[mlir][llvm] Add memcpy support for mem2reg/sroa.

[mlir][llvm] Add memcpy support for mem2reg/sroa.

This revision introduces SROA and mem2reg support for the family of
memcpy-like intrinsics (memcpy, memcpy.inline and memmove).

The mem2reg implementation transforms memcpys of full types into loads
and store. Memcpy between two promotable slots always disappear.

The SROA implementation transforms memcpys of *entire* aggregate types
into memcpys of all of their fields.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D152898