Add note that maybeHandleSimpleNomemIntrinsic may incorrectly match

horizontal/pairwise intrinsics

Author: thurstond

llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -3129,6 +3129,9 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
   /// fine).
   ///
   /// Caller guarantees that this intrinsic does not access memory.
+  ///
+  /// TODO: "horizontal"/"pairwise" intrinsics are often incorrectly matched by
+  ///       by this handler.
   [[maybe_unused]] bool
   maybeHandleSimpleNomemIntrinsic(IntrinsicInst &I,
                                   unsigned int trailingFlags) {