[llvm] AMDGPU GlobalISel Add64 support (PR #124763)

[Matt Arsenault via llvm-commits]

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@@ -730,13 +730,29 @@ AMDGPULegalizerInfo::AMDGPULegalizerInfo(const GCNSubtarget &ST_,
           .widenScalarToNextMultipleOf(0, 32)
           .maxScalar(0, S32);
     } else {
-      getActionDefinitionsBuilder({G_ADD, G_SUB})
+      getActionDefinitionsBuilder(G_SUB)
           .legalFor({S32, S16, V2S16})
           .clampMaxNumElementsStrict(0, S16, 2)
           .scalarize(0)
           .minScalar(0, S16)
           .widenScalarToNextMultipleOf(0, 32)
           .maxScalar(0, S32);
+      if (ST.hasLshlAddB64())
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arsenm wrote:

Can shrink this but it should be right 

https://github.com/llvm/llvm-project/pull/124763