[IndVars] Fix a subtle bug in optimizeLoopExits

[IndVars] Fix a subtle bug in optimizeLoopExits

The original code failed to account for the fact that one exit can have a pointer exit count without all of them having pointer exit counts.  This could cause two separate bugs:
1) We might exit the loop early, and leave optimizations undone.  This is what triggered the assertion failure in the reported test case.
2) We might optimize one exit, then exit without indicating a change.  This could result in an analysis invalidaton bug if no other transform is done by the rest of indvars.

Note that the pointer exit counts are a really fragile concept.  They show up only when we have a pointer IV w/o a datalayout to provide their size.  It's really questionable to me whether the complexity implied is worth it.

llvm-svn: 366829