[mlir][Transforms][NFC] Dialect conversion: Remove "finalize" phase (#116934)

The dialect conversion driver has three phases:
- **Create** `IRRewrite` objects as the IR is traversed.
- **Finalize** `IRRewrite` objects. During this phase, source
materializations for mismatching value types are created. (E.g., when
`Value` is replaced with a `Value` of different type, but there is a
user of the original value that was not modified because it is already
legal.)
- **Commit** `IRRewrite` objects. During this phase, all remaining IR
modifications are materialized. In particular, SSA values are actually
being replaced during this phase.

This commit removes the "finalize" phase. This simplifies the code base
a bit and avoids one traversal over the `IRRewrite` stack. Source
materializations are now built during the "commit" phase, right before
an SSA value is being replaced.

This commit also removes the "inverse mapping" of the conversion value
mapping, which was used to predict if an SSA value will be dead at the
end of the conversion. This check is replaced with an approximate check
that does not require an inverse mapping. (A false positive for `v` can
occur if another value `v2` is mapped to `v` and `v2` turns out to be
dead at the end of the conversion. This case is not expected to happen
very often.) This reduces the complexity of the driver a bit and removes
one potential source of bugs. (There have been bugs in the usage of the
inverse mapping in the past.)

`BlockTypeConversionRewrite` no longer stores a pointer to the type
converter. This pointer is now stored in `ReplaceBlockArgRewrite`.

This commit is in preparation of merging the 1:1 and 1:N dialect
conversion driver. It simplifies the upcoming changes around the
conversion value mapping. (API surface of the conversion value mapping
is reduced.)