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

[Kane Wang via llvm-commits]

================
@@ -4773,6 +4773,15 @@ LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT LowerHintTy) {
     return lowerVectorReduction(MI);
   case G_VAARG:
     return lowerVAArg(MI);
+  case G_ATOMICRMW_SUB: {
+    auto [Ret, RetLLT, Mem, MemLLT, Val, ValLLT] = MI.getFirst3RegLLTs();
----------------
ReVe1uv wrote:

Yes, after reusing the `Ret` register to avoid creating a new `copy` instruction, should just use `getFirst3Reg`.

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