[mlir][Transforms][NFC] Turn block type conversion into `IRRewrite` (#81756)

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 signature conversion of a block was only a "partial" IR
rewrite. Rollbacks were triggered via
`BlockTypeConversionRewrite::rollback`, but there was no
`BlockTypeConversionRewrite::commit` equivalent.

Overview of changes:
* Remove `ArgConverter`, an internal helper class that kept track of all
block type conversions. There is now a separate
`BlockTypeConversionRewrite` for each block type conversion.
* No more special handling for block type conversions. They are now
normal "IR rewrites", just like "block creation" or "block movement". In
particular, trigger "commits" of block type conversion via
`BlockTypeConversionRewrite::commit`.
* Remove `ArgConverter::notifyOpRemoved`. This function was used to
inform the `ArgConverter` that an operation was erased, to prevent a
double-free of operations in certain situations. It would be unpractical
to add a `notifyOpRemoved` API to `IRRewrite`. Instead, erasing
ops/block should go through a new `SingleEraseRewriter` (that is owned
by the `ConversionPatternRewriterImpl`) if there is chance of
double-free. This rewriter ignores `eraseOp`/`eraseBlock` if the
op/block was already freed.