[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)
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fmayer wrote:

If I only read this description, I'd have no idea what this does

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