[SCEV] A different fix for PR33494

Summary:
I don't think rL309080 is the right fix for PR33494 -- caching ExitLimit only
hides the problem[0].  The real issue is that because of how we forget SCEV
expressions ScalarEvolution::getBackedgeTakenInfo, in the test case for PR33494
computing the backedge for any loop invalidates the trip count for every other
loop.  This effectively makes the SCEV cache useless.

I've instead made the SCEV expression invalidation in
ScalarEvolution::getBackedgeTakenInfo less aggressive to fix this issue.

[0]: One way to think about this is that rL309080 essentially augmented the
backedge-taken-count cache with another equivalent exit-limit cache.  The bug
went away because we were explicitly not clearing the exit-limit cache in
getBackedgeTakenInfo.  But instead of doing all of that, we can just avoid
clearing the backedge-taken-count cache.

Reviewers: mkazantsev, mzolotukhin

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D39361

llvm-svn: 319678