[NewGVN] FIx phi-of-ops in the presence of memory read operations

The phi-of-ops functionality has a function OpIsSafeForPHIOfOps
to determine when it's safe to create the new phi.
But this function only checks for the obvious dominator conditions
and ignores memory.
This patch takes the conservative approach and disables phi-of-ops
whenever there's a load that doesn't dominate the phi, as its
value may be affected by a store inside the loop.

This can be improved later to check aliasing between the
load/stores.

Fixes https://llvm.org/PR53277

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D117999