Make identity cast operations with the same operand and result types legal

Make identity cast operations with the same operand and result types legal

    Instead, fold such operations. This way callers don't need to conditionally create cast operations depending on if a value already has the target type.

    Also, introduce areCastCompatible to allow cast users to verify that the generated op will be valid before creating the operation.

    TESTED with unit tests

--

PiperOrigin-RevId: 245606133