[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