[mlir][Transforms][NFC] Modularize block actions (#81237)

Throughout the rewrite process, the dialect conversion maintains a list
of "block actions" that can be rolled back upon failure. This commit
encapsulates the existing block actions into separate classes, making it
easier to add additional actions in the future.

This commit also renames "block actions" to "IR rewrites". In a
subsequent commit, an "operation rewrite" class that allows rolling back
movements of single operations is added. This is to support
`moveOpBefore` in the dialect conversion.

Rewrites have two methods: `commit()` commits an action. It can no
longer be rolled back afterwards. `rollback()` undoes a rewrite. It can
no longer be committed afterwards.