[mlir][Transforms][NFC] Simplify `BlockTypeConversionRewrite` (#83286)

When a block signature is converted during dialect conversion, a
`BlockTypeConversionRewrite` object is stored in the stack of rewrites.
Such an object represents multiple steps:
- Splitting the old block, i.e., creating a new block and moving all
operations over.
- Rewriting block arguments.
- Erasing the old block.

We have dedicated `IRRewrite` objects that represent "creating a block",
"moving an op" and "erasing a block". This commit reuses those rewrite
objects, so that there is less work to do in
`BlockTypeConversionRewrite::rollback` and
`BlockTypeConversionRewrite::commit`/`cleanup`.

Note: This change is in preparation of adding listener support to the
dialect conversion. The less work is done in a `commit` function, the
fewer notifications will have to be sent.