[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.

llvm-svn: 251770