[mlir][Transforms][NFC] Turn op/block arg replacements into IRRewrites

This commit is a refactoring of the dialect conversion. The dialect conversion maintains a list of "IR rewrites" that can be commited (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.

Author: matthias-springer