Implement lowering of element-wise fixed point add and mul to the standard dialect.

    This also does the following:
      - Removes the poc POT add implementation in favor of a version that does not rescale.
      - Adds a handful of FxpMathOps which are needed (these are for comment and we may want to move them to the StandardOps dialect).
      - Adds a canonicalizer to the StorageCastOp, which removes some cruft once conversions have been done.
      - Adds a couple of predicates to OpBase.

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PiperOrigin-RevId: 244287706