[deref-at-point] restrict inference of dereferenceability based on allocsize attribute

Support deriving dereferenceability facts from allocation sites with known object sizes while correctly accounting for any possibly frees between allocation and use site. (At the moment, we're conservative and only allowing it in functions where we know we can't free.)

This is part of the work on deref-at-point semantics. I'm making the change unconditional as the miscompile in this case is way too easy to trip by accident, and the optimization was only recently added (by me).

There will be a follow up patch wiring through TLI since that should now be doable without introducing widespread miscompiles.

Differential Revision: https://reviews.llvm.org/D95815