[MemCpyOpt] Teach memcpyopt to handle loads from the constant memory.

[MemCpyOpt] Teach memcpyopt to handle loads from the constant memory.

- Loads from the constant memory (either explicit one or as the source
  of memory transfer intrinsics) won't alias any stores.

Reviewed By: asbirlea, efriedma

Differential Revision: https://reviews.llvm.org/D107605