Reland "[mlir][mem2reg] Expose algorithm internals."

Reland "[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.

This commit reverts f333977eb20a and relands 91cff8a71872.

The original commit was reverted accidentally due to a misinterpretation
of a bazel build bot failure.

Reviewed By: gysit

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