[mlir][Transforms][NFC] Turn block type convertion into IRRewrite

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, 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.

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Author: matthias-springer