Implement lowering of quant.dcast to the fxpmathops and standard dialect.

    Note that I broke this out as a separate pass because intermediate transformations often produce qcast/dcast ops that are integral to the transformation, and it is typical to want to lower any remaining, unmatched casts at the end of quantization. If this flexibility ends up not being needed, they can be collapsed into the same pass. This is included in the same cpp file because all of the math ops will need to defer to emitting quantize/dequantize logic for cases that they cannot be fully lowered to fixed-point math.

    Also, the new convertistof op needs to be evaluated for inclusion in StandardOps.

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PiperOrigin-RevId: 244768679