Add FxpMathOps real_matmul and real_matmul_bias.

    Also:
      - cleans up some operand names for consistency
      - remove the broadcast_dims attribute as it isn't used
      - adds an IsNullAttr predicate which is needed to match optional clamp attributes on these kind of ops (needed to simplify some out of tree transforms on the new matmul op)

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PiperOrigin-RevId: 246370576