[mlir][Transforms][NFC] Turn op creation into `IRRewrite` (#81759)

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, the dialect conversion kept track of "op creation" in
separate internal data structures. This commit turns "op creation" into
an `IRRewrite` that can be committed and rolled back just like any other
rewrite. This commit simplifies the internal state of the dialect
conversion.