Add support for an analysis mode to DialectConversion.

This mode analyzes which operations are legalizable to the given target if a conversion were to be applied, i.e. no rewrites are ever performed even on success. This mode is useful for device partitioning or other utilities that may want to analyze the effect of conversion to different targets before performing it.

The analysis method currently just fills a provided set with the operations that were found to be legalizable. This can be extended in the future to capture more information as necessary.

PiperOrigin-RevId: 259987105