set the cost of tiny trees to INT_MAX in SLP vectorizer to disable vectorization on them

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191314 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -947,7 +947,7 @@ int BoUpSLP::getTreeCost() {
     if (!VectorizableTree.size()) {
       assert(!ExternalUses.size() && "We should not have any external users");
     }
-    return 0;
+    return INT_MAX;
   }
 
   unsigned BundleWidth = VectorizableTree[0].Scalars.size();