swift234

[simplify-cfg] Teach simplify cfg how to split struct/tuple BB arguments.

The reason to do this is that a struct/tuple with multiple non-trivial
fields does not have the same RC identity as its fields. This is b/c a
retain_value on the struct/tuple acts as multiple reference counting
operations in such a case implying we must be conservative.

The two ways to solve this are:

1. To disassociate RC identity solely from SSA values to reasoning about
SSA values and components of those values. This would add a lot of
complication.
2. We can always split up the aggregates in such cases.

We have already taken the approach of 2 in the guise of function
signature optimization.

This commit performs the same operation on basic block
arguments (i.e. phi nodes). We only perform the operation if we have
more than 1 non-trivial field. If we find reason to do it in all cases,
we can extend the optimization.

The changes on the perf test suite >= ~10% that I saw:

Legend:
	Left: before.
	Right: after.
--------------------------------------------------------------------------------

TITLE                      LEFT        RIGHT        RIGHT - LEFT        RIGHT/LEFT
Life                       1118.00     738.00       -380.00             0.66
DictionaryHashableClass    34469.00    24465.00     -10004.00           0.71
Chars                      1163.00     1051.00      -112.00             0.90
RecursiveOwnedParameter    2082.00     2265.00      183.00              1.09
DeadArray                  160.00      184.00       24.00               1.15

The dead array regression is phantom since the actual object files are the same
and the recursive owned parameter seems to be caused by small register allocation
changes.

Swift SVN r31756