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

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

This PR is needed by https://github.com/llvm/llvm-project/pull/184603 or we will get an error because the `Two-Address instruction pass` gets the wrong commutable operands.

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