[llvm] [GlobalISel] Add `G_EXTRACT_VECTOR_ELT` for `computeKnownBits` (PR #164825)

[Matt Arsenault via llvm-commits]

================
@@ -643,6 +643,40 @@ void GISelValueTracking::computeKnownBitsImpl(Register R, KnownBits &Known,
     Known.Zero.setBitsFrom(LowBits);
     break;
   }
+  case TargetOpcode::G_EXTRACT_VECTOR_ELT: {
+    GExtractVectorElement &Extract = cast<GExtractVectorElement>(MI);
+    Register InVec = Extract.getVectorReg();
+    Register EltNo = Extract.getIndexReg();
+
+    auto ConstEltNo = getIConstantVRegVal(EltNo, MRI);
+
+    LLT VecVT = MRI.getType(InVec);
+    // computeKnownBits not yet implemented for scalable vectors.
+    if (VecVT.isScalableVector())
+      break;
+
+    const unsigned EltBitWidth = VecVT.getScalarSizeInBits();
+    const unsigned NumSrcElts = VecVT.getNumElements();
+    // A return type different from the vector's element type may lead to
+    // issues with pattern selection. Bail out to avoid that.
+    if (BitWidth > EltBitWidth) {
+      break;
+    }
+
+    Known.Zero.setAllBits();
+    Known.One.setAllBits();
+
+    // If we know the element index, just demand that vector element, else for
+    // an unknown element index, ignore DemandedElts and demand them all.
+    APInt DemandedSrcElts = APInt::getAllOnes(NumSrcElts);
+    if (ConstEltNo && ConstEltNo->ult(NumSrcElts))
+      DemandedSrcElts =
+          APInt::getOneBitSet(NumSrcElts, ConstEltNo->getZExtValue());
+
+    computeKnownBitsImpl(InVec, Known, DemandedSrcElts, Depth + 1);
+
----------------
arsenm wrote:

```suggestion
```

https://github.com/llvm/llvm-project/pull/164825