Preserve aliasing info during memory intrinsics lowering

By specification, source and destination of llvm.memcpy.* must either be equal or non-overlapping. This semantics is hard or impossible to figure out once lowered. This patch explicitly marks loads from source and stores to destination as not aliasing if source and destination is known to be not equal.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D118441