[GVN] Invalidate MDA when deduplicating phi nodes

Duplicate phi nodes were being directly removed, without
invalidating MDA. This could result in a new phi node being
allocated at the same address, incorrectly reusing a cache entry.

Fix this by optionally allowing EliminateDuplicatePHINodes() to
collect phi nodes to remove into a vector, which allows GVN to
handle removal itself.

Fixes https://github.com/llvm/llvm-project/issues/64598.

Differential Revision: https://reviews.llvm.org/D158849