[llvm] [AMDGPU] Elide bitcast fold i64 imm to build_vector (PR #154115)

[Matt Arsenault via llvm-commits]

================
@@ -14584,13 +14584,39 @@ SITargetLowering::performExtractVectorEltCombine(SDNode *N,
     return V;
   }
 
+  // EXTRACT_VECTOR_ELT (v2i32 bitcast (i64/f64:k), Idx)
+  //   =>
+  // i32:Lo(k) if Idx == 0, or
+  // i32:Hi(k) if Idx == 1
+  auto *Idx = dyn_cast<ConstantSDNode>(N->getOperand(1));
+  if (Vec.getOpcode() == ISD::BITCAST && VecVT == MVT::v2i32 && Idx) {
+    SDLoc SL(N);
+    SDValue PeekThrough = peekThroughBitcasts(Vec);
+    auto *KImm = dyn_cast<ConstantSDNode>(PeekThrough);
+    if (KImm && KImm->getValueType(0).getSizeInBits() == 64) {
+      uint64_t KImmValue = KImm->getZExtValue();
+      if (Idx->getZExtValue() == 0)
+        return DAG.getConstant(Lo_32(KImmValue), SL, MVT::i32);
+      else
+        return DAG.getConstant(Hi_32(KImmValue), SL, MVT::i32);
----------------
arsenm wrote:

```suggestion
        return DAG.getConstant(KImmValue >> (32 * Idx->getZExtValue()), SL, MVT::i32);
```

No else after return 

https://github.com/llvm/llvm-project/pull/154115