swift234

SimplifyCFG: Implement general jumpthreading using the SSAUpdater

simplifyInstruction conciously does not create constants (AFAIK) so we need to
run instruction combine after simplify-cfg to enable more cfg simplification
exposed by jumpthreading. We can revisit this decision in a follow-up commit if
necessary (I believe it to be useful for simplifyInstruction to be able to
create constants and for simplifycfg to use simplifyInstruction on the branch
condition).

O:
benchmark      ,  baserun0  ,  optrun0  ,  delta,  speedup
Fibonacci      ,  1473.00   ,  1317.00  ,  75.00   ,        5.8%
Histogram      ,  407.00    ,  390.00   ,  23.00   ,        6.1%
InsertionSort  ,  1273.00   ,  1200.00  ,  79.00   ,        6.7%
Life           ,  74.00     ,  69.00    ,  4.00    ,        5.8%
NestedLoop     ,  937.00    ,  883.00   ,  56.00   ,        6.4%
R17315246      ,  8.00      ,  801.00   ,  793.00  ,        -99.0%
SelectionSort  ,  1150.00   ,  921.00   ,  226.00  ,        24.5%

Ounchecked:
Histogram      ,  394.00    ,  342.00   ,  51.00   ,        15.0%
InsertionSort  ,  1122.00   ,  1024.00  ,  85.00   ,        8.3%
Life           ,  57.00     ,  44.00    ,  9.00    ,        20.9%
SelectionSort  ,  1312.00   ,  1060.00  ,  246.00  ,        23.3%

The R17315246 regression is somewhat bad. We dependent on a loop form such that
LLVM transforms the loop into an inner loop that just iterates from x to y and
this is unrolled (good version) the slow version has a loop with cond_fail
control flow and is not unrolled. The loop does nothing more than count up.
I have not being able to narrow this down further.

rdar://16821595

Swift SVN r21124