[llvm] [AMDGPU] Create AMDGPUMnemonicAlias tablegen class (PR #89288)

[Joe Nash via llvm-commits]

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@@ -1073,10 +1072,18 @@ multiclass MIMG_Atomic_Addr_Helper_m <mimgopc op, string asm,
         if !empty(renamed) then
           def _V4_gfx12 : VIMAGE_Atomic_gfx12 <op, asm, data_rc, 4, enableDasm>;
         else
-          def _V4_gfx12 : VIMAGE_Atomic_gfx12_Renamed <op, asm, renamed, data_rc, 4, enableDasm>;
+          def _V4_gfx12 : VIMAGE_Atomic_gfx12_Renamed <op, renamed, data_rc, 4, enableDasm>;
       }
     }
   }
+  if !and(op.HAS_GFX12, !not(!empty(renamed))) then
+    def : AMDGPUMnemonicAlias<asm, renamed> {
+      let AssemblerPredicate = isGFX12Plus;
+      let OtherPredicates = [HasImageInsts];
+      bit IsAtomicRet; // Unused
+      MIMGBaseOpcode BaseOpcode; // Unused
+      int VDataDwords; // Unused
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Sisyph wrote:

Same reason as we use letDummies. The fields are set in an outer let on on MIMG_Atomic_Addr_Helper_m. I considered pushing this alias down a level in the class hierarchy, and also pushing it up one level. But neither option seemed cleaner.

https://github.com/llvm/llvm-project/pull/89288