[llvm] [RISCV] Add findCommutedOpIndices support for Zvabd (PR #184602)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> > This PR is needed by #184603 or we will get an error because the `Two-Address instruction pass` gets the wrong commutable operands.
> 
> This is presumably because we set `isCommutable` on the Pseudos/Instructions already?

Yes, we have set `isCommutable` but we didn't override `findCommutedOpIndices`. It should be done at the beginning.

https://github.com/llvm/llvm-project/pull/184602