Refactor DialectConversion to use 'materializeConversion' when a type...

Refactor DialectConversion to use 'materializeConversion' when a type conversion must persist after the conversion has finished.

During conversion, if a type conversion has dangling uses a type conversion must persist after conversion has finished to maintain valid IR. In these cases, we now query the TypeConverter to materialize a conversion for us. This allows for the default case of a full conversion to continue working as expected, but also handle the degenerate cases more robustly.

PiperOrigin-RevId: 255637171