[MLIR][Mem2Reg] Replace pattern based approach with a bulk one (#85426)

This commit changes MLIR's Mem2Reg implementation back from being
pattern based into a full pass. Using Mem2Reg as a pattern is
wasteful, as each application can invalidate the dominance info.
Applying changes in bulk allows for reuse of the same dominance info.

Unfortunately, this requires some test changes, due to the `IRBuilder`
not simplifying IR.