[MLIR][Mem2Reg] Improve performance by avoiding recomputations (#91444)

This commit ensures that Mem2Reg reuses the `DominanceInfo` as well as
block index maps to avoid expensive recomputations. Due to the recent
migration to `OpBuilder`, the promotion of a slot does no longer replace
blocks. Having stable blocks makes the `DominanceInfo` preservable and
additionally allows to cache block index maps between different
promotions.

Performance measurements on very large functions show an up to 4x
speedup by these changes.