swift234

Sink retains down in the current BB if they cannot be sunk to a successor.

We currently only sink retains to successor BBs.

However, if a retain can't be moved to a successor, we may still want to move it as late in the current BB as
possible.  For example, we want retains after called to _swift_isUniquelyReferenced and not before it.

Here are performance numbers for -O.  Note that i'm about to commit improvements to enum_is_tag which fixes the
regressions here.

Ackermann``````,``````````1413.00```,`````````````````1566.00```,``````````153.00``,`````````````````-9.8%
ArrayLiteral```,``````````834.00````,`````````````````859.00````,``````````25.00```,`````````````````-2.9%
Ary````````````,``````````1110.00```,`````````````````1121.00```,``````````11.00```,`````````````````-1.0%
Ary2```````````,``````````1092.00```,`````````````````1105.00```,``````````13.00```,`````````````````-1.2%
Ary3```````````,``````````1164.00```,`````````````````1193.00```,``````````29.00```,`````````````````-2.4%
DeltaBlue``````,``````````2195.00```,`````````````````2204.00```,``````````9.00````,`````````````````-0.4%
Dictionary`````,``````````502.00````,`````````````````512.00````,``````````10.00```,`````````````````-2.0%
Dictionary2````,``````````884.00````,`````````````````895.00````,``````````11.00```,`````````````````-1.2%
Dictionary3````,``````````859.00````,`````````````````870.00````,``````````11.00```,`````````````````-1.3%
EditDistance```,``````````1344.00```,`````````````````1344.00```,``````````0.00````,`````````````````0.0%
Fibonacci``````,``````````1382.00```,`````````````````1309.00```,``````````73.00```,`````````````````5.6%
ForLoops```````,``````````1318.00```,`````````````````1324.00```,``````````6.00````,`````````````````-0.5%
Forest`````````,``````````757.00````,`````````````````775.00````,``````````18.00```,`````````````````-2.3%
GlobalClass````,``````````1617.00```,`````````````````1620.00```,``````````3.00````,`````````````````-0.2%
Hash```````````,``````````660.00````,`````````````````718.00````,``````````58.00```,`````````````````-8.1%
HeapSort```````,``````````1256.00```,`````````````````1292.00```,``````````36.00```,`````````````````-2.8%
Histogram``````,``````````313.00````,`````````````````296.00````,``````````17.00```,`````````````````5.7%
InsertionSort``,``````````1223.00```,`````````````````1351.00```,``````````128.00``,`````````````````-9.5%
Life```````````,``````````69.00`````,`````````````````70.00`````,``````````1.00````,`````````````````-1.4%
LinkedList`````,``````````1516.00```,`````````````````1529.00```,``````````13.00```,`````````````````-0.9%
MatMul`````````,``````````224.00````,`````````````````239.00````,``````````15.00```,`````````````````-6.3%
Memset`````````,``````````43.00`````,`````````````````43.00`````,``````````0.00````,`````````````````0.0%
MonteCarloE````,``````````905.00````,`````````````````900.00````,``````````5.00````,`````````````````0.6%
MonteCarloPi```,``````````643.00````,`````````````````654.00````,``````````11.00```,`````````````````-1.7%
NBody``````````,``````````42.00`````,`````````````````41.00`````,``````````1.00````,`````````````````2.4%
NestedLoop`````,``````````931.00````,`````````````````876.00````,``````````55.00```,`````````````````6.3%
NopDeinit``````,``````````1119.00```,`````````````````1117.00```,``````````2.00````,`````````````````0.2%
Phonebook``````,``````````1424.00```,`````````````````1395.00```,``````````29.00```,`````````````````2.1%
PrimeNum```````,``````````214.00````,`````````````````219.00````,``````````5.00````,`````````````````-2.3%
Prims``````````,``````````1182.00```,`````````````````1198.00```,``````````16.00```,`````````````````-1.3%
QuickSort``````,``````````1196.00```,`````````````````1166.00```,``````````30.00```,`````````````````2.6%
R17315246``````,``````````798.00````,`````````````````798.00````,``````````0.00````,`````````````````0.0%
RC4````````````,``````````25.00`````,`````````````````25.00`````,``````````0.00````,`````````````````0.0%
RIPEMD`````````,``````````724.00````,`````````````````746.00````,``````````22.00```,`````````````````-2.9%
Random`````````,``````````960.00````,`````````````````963.00````,``````````3.00````,`````````````````-0.3%
Rectangles`````,``````````836.00````,`````````````````849.00````,``````````13.00```,`````````````````-1.5%
Richards```````,``````````220.00````,`````````````````225.00````,``````````5.00````,`````````````````-2.2%
SelectionSort``,``````````931.00````,`````````````````932.00````,``````````1.00````,`````````````````-0.1%
SmallPT````````,``````````1014.00```,`````````````````1039.00```,``````````25.00```,`````````````````-2.4%
StdlibSort`````,``````````1617.00```,`````````````````1602.00```,``````````15.00```,`````````````````0.9%
StrCat`````````,``````````1061.00```,`````````````````1121.00```,``````````60.00```,`````````````````-5.4%
StrSplitter````,``````````1607.00```,`````````````````1533.00```,``````````74.00```,`````````````````4.8%
StrToInt```````,``````````616.00````,`````````````````612.00````,``````````4.00````,`````````````````0.7%
StringBuilder``,``````````701.00````,`````````````````682.00````,``````````19.00```,`````````````````2.8%
StringWalk`````,``````````1165.00```,`````````````````1182.00```,``````````17.00```,`````````````````-1.4%
Totals`````````,``````````42392.00``,`````````````````42778.00``,``````````386.00``,`````````````````-0.9%
TwoSum`````````,``````````581.00````,`````````````````564.00````,``````````17.00```,`````````````````3.0%
Walsh``````````,``````````105.00````,`````````````````104.00````,``````````1.00````,`````````````````1.0%

Swift SVN r21133