[RISCV] Add isCommutable for pseudos without merge operand

`vmadc` and vector mask-register logical instructions are commutable
and there is no merge operand for their pseudos.

We add `isCommutable=1` for these pseudos to gain more optimization
opportunities.

This patch fixes part of https://github.com/llvm/llvm-project/issues/64422.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D158976