[LAA] We only need pointer checks if there are non-zero checks (NFC).

If it turns out that we can do runtime checks, but there are no
runtime-checks to generate, set RtCheck.Need to false.

This can happen if we can prove statically that the pointers passed in
to canCheckPtrAtRT do not alias. This should not change any results, but
allows us to skip some work and assert that runtime checks are
generated, if LAA indicates that runtime checks are required.

Reviewers: anemet, Ayal

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D79969

Note: This is a recommit of 259abfc7,
with some suggested renaming.