[RISCV] Replace RISCVISD::GREV/GORC/SHFL/UNSHFL with BREV8/ORC_B/ZIP/UNZIP.

With Zbp removed, we no longer need the generalized forms.

The computeKnownBitsForTargetNode code brev8/orc.b is still based
on the general form with the shift amount forced to 7.