[llvm] [RISCV][GlobalISel] Legalize and select G_ATOMICRMW_ADD instruction (PR #153791)

[Kane Wang via llvm-commits]

ReVe1uv wrote:

Hi @topperc  — I can’t assign reviewers on this repo, so I’m tagging you here for a review.
This PR add instruction selection and legalization support for G_ATOMICRMW_ADD on RISC-V

https://github.com/llvm/llvm-project/pull/153791