[SLP]Skip buildvector tree, if all scalars are used externally and remain scalar

If the buildvector is going to be vector with threshold cost < 0 and all
buildvector scalars are externally used and remain scalar, such a tree
should not be vectorized, it may lead to a compiler hang because same
scalars remain in the function and will be vectorized once again.

Fixes #172609

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -15617,6 +15617,17 @@ bool BoUpSLP::isTreeTinyAndNotFullyVectorizable(bool ForReduction) const {
          allConstant(VectorizableTree[1]->Scalars))))
     return true;
 
+  // Buildvector with externally used scalars, which should remain as scalars,
+  // should not be vectorized, the compiler may hang.
+  if (SLPCostThreshold < 0 && VectorizableTree.size() > 1 &&
+      isa<InsertElementInst>(VectorizableTree[0]->Scalars[0]) &&
+      VectorizableTree[1]->hasState() &&
+      VectorizableTree[1]->State == TreeEntry::Vectorize &&
+      all_of(VectorizableTree[1]->Scalars, [&](Value *V) {
+        return ExternalUsesAsOriginalScalar.contains(V);
+      }))
+    return true;
+
   // If the graph includes only PHI nodes and gathers, it is defnitely not
   // profitable for the vectorization, we can skip it, if the cost threshold is
   // default. The cost of vectorized PHI nodes is almost always 0 + the cost of