[llvm] [ConstantFolding] Add folding for [de]interleave2, insert and extract (PR #141301)

[Luke Lau via llvm-commits]

================
@@ -3734,6 +3738,98 @@ static Constant *ConstantFoldFixedVectorCall(
     }
     return nullptr;
   }
+  case Intrinsic::vector_extract: {
+    auto *Vec = dyn_cast<Constant>(Operands[0]);
+    auto *Idx = cast<ConstantInt>(Operands[1]);
+    if (!Vec || !Idx || !isa<FixedVectorType>(Vec->getType()))
+      return nullptr;
+
+    unsigned NumElements = FVTy->getNumElements();
+    unsigned VecNumElements =
+        cast<FixedVectorType>(Vec->getType())->getNumElements();
+    unsigned StartingIndex = Idx->getZExtValue();
+
+    // Extracting entire vector is nop
+    if (NumElements == VecNumElements && StartingIndex == 0)
+      return Vec;
+
+    const unsigned NonPoisonNumElements =
+        std::min(StartingIndex + NumElements, VecNumElements);
+    for (unsigned I = StartingIndex; I < NonPoisonNumElements; ++I) {
+      Constant *Elt = Vec->getAggregateElement(I);
+      if (!Elt)
+        return nullptr;
+      Result[I - StartingIndex] = Elt;
+    }
----------------
lukel97 wrote:

Stylistic nit, you could avoid `NonPoisonNumElements` and the second loop if you handle it in the main loop
```suggestion
    for (unsigned I = 0; I < NumElements; ++I) {
      // Out of bounds elements are poison
      if (StartingIndex + I >= VecNumElements) {
        Result[I] = PoisonValue::get(FVTy->getElementType());
        continue;
      }
      Constant *Elt = Vec->getAggregateElement(StartingIndex + I);
      if (!Elt)
        return nullptr;
      Result[I] = Elt;
    }
```

https://github.com/llvm/llvm-project/pull/141301