[SROA] Allow SROA on pointers with invariant group intrinsic uses

When we are able to SROA an alloca, we know all uses of it, meaning we
don't have to preserve the invariant group intrinsics and metadata.

It's possible that we could lose information regarding redundant
loads/stores, but that's unlikely to have any real impact since right
now the only user is Clang and vtables.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D99760