[SLP][NFC]Unify add() member function in CostEstimator, NFC.

Make add() function smart enough to understand that the shuffle of
a single entry is requested, if it sees that the second node is the same
as the first.

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -7327,6 +7327,15 @@ class BoUpSLP::ShuffleCostEstimator : public BaseShuffleAnalysis {
     return VecBase;
   }
   void add(const TreeEntry *E1, const TreeEntry *E2, ArrayRef<int> Mask) {
+    if (E1 == E2) {
+      assert(all_of(Mask,
+                    [=](int Idx) {
+                      return Idx < static_cast<int>(E1->getVectorFactor());
+                    }) &&
+             "Expected single vector shuffle mask.");
+      add(E1, Mask);
+      return;
+    }
     CommonMask.assign(Mask.begin(), Mask.end());
     InVectors.assign({E1, E2});
   }
@@ -7525,10 +7534,7 @@ BoUpSLP::getEntryCost(const TreeEntry *E, ArrayRef<Value *> VectorizedVals,
       LLVM_DEBUG(dbgs() << "SLP: shuffled " << Entries.size()
                         << " entries for bundle "
                         << shortBundleName(VL) << ".\n");
-      if (Entries.size() == 1)
-        Estimator.add(Entries.front(), Mask);
-      else
-        Estimator.add(Entries.front(), Entries.back(), Mask);
+      Estimator.add(Entries.front(), Entries.back(), Mask);
       if (all_of(GatheredScalars, PoisonValue ::classof))
         return Estimator.finalize(E->ReuseShuffleIndices);
       return Estimator.finalize(