[SLP][NFC]Exit early from attempt-to-reorder, if it is useless

Adds early exits, which just save compile time. It can exit earl, if the
total number of scalars is 2, or all scalars are constant, or the opcode
is the same and not alternate. In this case reordering will not happen
and compiler can exit early to save compile time

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -9362,6 +9362,12 @@ void BoUpSLP::reorderGatherNode(TreeEntry &TE) {
   DenseMap<std::pair<size_t, Value *>, SmallVector<LoadInst *>> LoadsMap;
   SmallSet<size_t, 2> LoadKeyUsed;
 
+  // Do not reorder nodes if it small (just 2 elements), all-constant or all
+  // instructions have same opcode already.
+  if (TE.Scalars.size() == 2 || (TE.getOpcode() && !TE.isAltShuffle()) ||
+      all_of(TE.Scalars, isConstant))
+    return;
+
   if (any_of(seq<unsigned>(TE.Idx), [&](unsigned Idx) {
         return VectorizableTree[Idx]->isSame(TE.Scalars);
       }))