Refactor the DialectConversion process to clone each function and then...

    Refactor the DialectConversion process to clone each function and then operate in-place, as opposed to incrementally constructing a new function. This is crucial to allowing the use of non type-conversion patterns(normal RewritePatterns) as part of the conversion process.

    The converter now works by inserting fake producer operations when replacing the results of an existing operation with values of a different, now legal, type. These fake operations are guaranteed to never escape the converter.

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PiperOrigin-RevId: 248969130