[llvm] [NFCI][msan] Refactor into 'horizontalReduce' (PR #152961)

[Florian Mayer via llvm-commits]

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@@ -2690,6 +2690,44 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     SC.Done(&I);
   }
 
+ // Perform a bitwise OR on the horizontal pairs (or other specified grouping)
+  // of elements. This is convenient for instrumenting horizontal add/sub.
+  Value *horizontalReduce(IntrinsicInst &I, unsigned ReductionFactor,
+                          Value *VectorA, Value* VectorB) {
+    assert(isa<FixedVectorType>(VectorA->getType()));
+    unsigned TotalNumElems
+      = cast<FixedVectorType>(VectorA->getType())->getNumElements();
+
+    if (VectorB) {
+      assert(VectorA->getType() == VectorB->getType());
+      TotalNumElems = TotalNumElems * 2;
+    }
+
+    assert(TotalNumElems % ReductionFactor == 0);
+
+    Value *Or = nullptr;
+
+    IRBuilder<> IRB(&I);
+    for (unsigned i = 0; i < ReductionFactor; i++) {
+      SmallVector<int, 16> Mask;
+      for (unsigned X = 0; X < TotalNumElems; X += ReductionFactor)
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fmayer wrote:

I would put an example in a comment 

https://github.com/llvm/llvm-project/pull/152961