[RISCV] Remove RISCVISD::WMACC*. Match during isel. NFC (#181197)

I think we may want to be able to fold ADDD nodes independent of the MUL
in some cases. For example turning NSRAI into NSRARI.

If we fold ADDD into WMACC we would need to be able to extract it again.
Keep the nodes separate avoids this.

Code change was assisted by AI.

GitOrigin-RevId: a809d6409f7a4785d0301bddea8a1ce6713c6179