[NewGVN][PHIOFOPS] Relax conditions when checking safety of memory accesses

Currently, we consider any instruction that might read from memory to be unsafe for phi-of-ops.
This patch refines that by walking the clobbering memDefs until we either hit a block that
strictly dominates the phi block (safe) or we hit a clobbering memPhi (unsafe).

Differential Revision: https://reviews.llvm.org/D156055