Dialect Conversion: convert regions of operations when cloning them

Dialect conversion currently clones the operations that did not match any
pattern.  This includes cloning any regions that belong to these operations.
Instead, apply conversion recursively to the nested regions.

Note that if an operation matched one of the conversion patterns, it is up to
the pattern rewriter to fill in the regions of the converted operation.  This
may require calling back to the converter and is left for future work.

PiperOrigin-RevId: 240872410