[SCEV] Fix PR25369

Have `getConstantEvolutionLoopExitValue` work correctly with multiple
entry loops.

As far as I can tell, `getConstantEvolutionLoopExitValue` never did the
right thing for multiple entry loops; and before r249712 it would
silently return an incorrect answer.  r249712 changed SCEV to fail an
assert on a multiple entry loop, and this change fixes the underlying
issue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251770 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed