[llvm] [GlobalISel] Add G_EXTRACT_VECTOR_ELT to computeNumSignBits (PR #218283)

[Matt Arsenault via llvm-commits]

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@@ -2669,6 +2669,21 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
     }
     break;
   }
+  case TargetOpcode::G_EXTRACT_VECTOR_ELT: {
+    GExtractVectorElement &Extract = cast<GExtractVectorElement>(MI);
+    Register InVec = Extract.getVectorReg();
+    Register EltNo = Extract.getIndexReg();
+    LLT VecVT = MRI.getType(InVec);
+    if (VecVT.isScalableVector())
+      break;
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arsenm wrote:

This doesn't need to give up on scalable vectors, it can return the known fro the whole vector source 

https://github.com/llvm/llvm-project/pull/218283