During PHI elimination, split critical edges that move copies out of loops.

During PHI elimination, split critical edges that move copies out of loops.

This prevents the behavior observed in llvm.org/PR22369. I am not sure
whether I am reading the code correctly, but the early exit based on
isLiveOutPastPHIs() seems to make the wrong assumption that
RegisterCoalescer won't be able to coalesce those copies later.

This change hides the new behavior behind -no-phi-elim-live-out-early-exit
as it currently breaks four tests:
 * Assertion in:
     CodeGen/Hexagon/hwloop-cleanup.ll
 * Worse code in:
     CodeGen/X86/coalescer-commute4.ll
     CodeGen/X86/phys_subreg_coalesce-2.ll
     CodeGen/X86/zlib-longest-match.ll
   The root cause here seems to be that the heuristic that determines
   the visitation order in RegisterCoalescer gets less lucky.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231064 91177308-0d34-0410-b5e6-96231b3b80d8