[mlir][mem2reg] Add support for mem2reg in MemRef.

This patch implements the mem2reg interfaces for MemRef types. This only supports scalar memrefs of a small list of types. It would be beneficial to create more interfaces for default values before expanding support to more types. Additionally, I am working on an upcoming revision to bring SROA to MLIR that should help with non-scalar memrefs.

Reviewed By: gysit, Mogball

Differential Revision: https://reviews.llvm.org/D149441