[LAA] Revert a small part of r239295

This commit ([LAA] Fix estimation of number of memchecks) regressed the
logic a bit.  We shouldn't quit the analysis if we encounter a pointer
without known bounds *unless* we actually need to emit a memcheck for
it.

The original code was using NumComparisons which is now computed
differently.  Instead I compute NeedRTCheck from NumReadPtrChecks and
NumWritePtrChecks.

As side note, I find the separation of NeedRTCheck and CanDoRT
confusing, so I will try to merge them in a follow-up patch.

llvm-svn: 241756