swift234

New SIL-Opt pass: CopyForwarding (disabled, NFC).

This should hugely improve performance in generic code. However, it
exposes a downstream issue with phi node generation.

e.g.
PHI node has multiple entries for the same basic block with different incoming values!
  %497 = phi i64 [ %177, %570 ], [ 0, %570 ], [ %494, %492 ], [ %493, %492 ]
label %492
  %494 = phi i64 [ %177, %547 ], [ %402, %491 ]
  %493 = phi i64 [ %558, %547 ], [ %489, %491 ]

Swift SVN r21965