[MemorySSA] Support invariant.group metadata

The implementation is mostly copied from MemDepAnalysis. We want to look
at all loads and stores to the same pointer operand. Bitcasts and zero
GEPs of a pointer are considered the same pointer value. We choose the
most dominating instruction.

Since updating MemorySSA with invariant.group is non-trivial, for now
handling of invariant.group is not cached in any way, so it's part of
the walker. The number of loads/stores with invariant.group is small for
now anyway. We can revisit if this actually noticeably affects compile
times.

To avoid invariant.group affecting optimized uses, we need to have
optimizeUsesInBlock() not use invariant.group in any way.

Co-authored-by: Piotr Padlewski <prazek@google.com>

Reviewed By: asbirlea, nikic, Prazek

Differential Revision: https://reviews.llvm.org/D109134