[RISCV] Add isCommutable to vector pseudos like VADD

[topperc]

MachineCSE can commute instructions to find more CSE opportunities. We should make the vector pseudos commutable to enable this. We probably need custom handling in commuteInstructionImpl to skip the merge operand.

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

[wangpc-pp]

I thought we have added isCommutable to these pseudos since I remember that I saw the word Commutable before. But after investigation I just found that I was wrong, we only have added isCommutable to Multiply-Add Instructions.
Yeah, I think it worths doing.

[wangpc-pp]

One candidate patch: https://reviews.llvm.org/D158976