[mlir][Transforms][NFC] Turn op/block arg replacements into IRRewrites (#81757)

This commit is a refactoring of the dialect conversion. The dialect
conversion maintains a list of "IR rewrites" that can be committed (upon
success) or rolled back (upon failure).

Until now, op replacements and block argument replacements were kept
track in separate data structures inside the dialect conversion. This
commit turns them into `IRRewrite`s, so that they can be committed or
rolled back just like any other rewrite. This simplifies the internal
state of the dialect conversion.

Overview of changes:
* Add two new rewrite classes: `ReplaceBlockArgRewrite` and
`ReplaceOperationRewrite`. Remove the `OpReplacement` helper class; it
is now part of `ReplaceOperationRewrite`.
* Simplify `RewriterState`: `numReplacements` and `numArgReplacements`
are no longer needed. (Now being kept track of by `numRewrites`.)
* Add `IRRewrite::cleanup`. Operations should not be erased in `commit`
because they may still be referenced in other internal state of the
dialect conversion (`mapping`). Detaching operations is fine.
* `trackedOps` are now updated during the "commit" phase instead of
after applying all rewrites.

Author: matthias-springer