swift234

DominatorBasedJumpThreading: Resurrected

Teach dominator based simplifications to also thread dominated edges.

The code now handles cond_br and switch_enum terminators for both value based
simplifications (where the use is dominated) and jump threading edges (the edge
is dominated).

Update simplify_cfg.sil test cases for split edges.

This also handles the test case from rdar://20390647.

Swift SVN r27843