[RISCV] SMUL_LOHI/UMUL_LOHI should expand for RVV.

These and MULHS/MULHU both default to Legal. Targets need to set
the ones they don't support to Expand.

I think MULHS/MULHU likely has priority in most places so this
change probably isn't directly testable. I found it while looking
at disabling MULHS/MULHU for nxvXi64 as required for Zve64x.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D113325