[mlir][mem2reg] Expose algorithm internals.

This patch refactors the Mem2Reg infrastructure. It decouples
analysis from promotion, allowing for more control over the execution of
the logic. It also adjusts the interfaces to be less coupled to mem2reg
and more general. This will be useful for an upcoming revision
introducing generic SROA.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D149825