[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #123360)

[Simon Pilgrim via llvm-commits]

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@@ -9797,6 +10119,13 @@ void BoUpSLP::transformNodes() {
       reorderGatherNode(E);
   }
 
+  bool ForceLoadGather =
+      count_if(VectorizableTree, [](const std::unique_ptr<TreeEntry> &TE) {
+        return TE->isGather() && TE->hasState() &&
+               TE->getOpcode() == Instruction::Load &&
+               TE->getVectorFactor() < 16;
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RKSimon wrote:

OK - please add a comment describing the magic number

https://github.com/llvm/llvm-project/pull/123360