[llvm] [RISCV][GlobalISel] Lower G_ATOMICRMW_SUB via G_ATOMICRMW_ADD (PR #155972)

[Kane Wang via llvm-commits]

ReVe1uv wrote:

Hi @arsenm , could you please take another look at this PR? Also, since I don’t have commit access, I’d appreciate your help with merging it once it’s approved. Thanks!

https://github.com/llvm/llvm-project/pull/155972