LowerMemIntrinsics: Check address space aliasing for memmove expansion

For cases where we cannot insert an addrspacecast, we can still expand
like a memcpy if we know the address spaces cannot alias. Normally
non-aliasing memmoves are optimized to memcpy, but we cannot rely on
that for lowering. If a target has aliasing address spaces that cannot
be casted between, we still have to give up lowering this.