[llvm] [SLP][REVEC] Fix cost model for getGatherCost with FixedVectorType ScalarTy. (PR #109369)

[Alexey Bataev via llvm-commits]

================
@@ -11767,14 +11765,27 @@ InstructionCost BoUpSLP::getGatherCost(ArrayRef<Value *> VL, bool ForPoisonSrc,
     }
 
     DuplicateNonConst = true;
-    ShuffledElements.setBits(I * ScalarTyNumElements,
-                             I * ScalarTyNumElements + ScalarTyNumElements);
+    ShuffledElements.setBit(I);
     ShuffleMask[I] = Res.first->second;
   }
-  if (ForPoisonSrc)
-    Cost =
-        TTI->getScalarizationOverhead(VecTy, ~ShuffledElements, /*Insert*/ true,
-                                      /*Extract*/ false, CostKind);
+  if (ForPoisonSrc) {
+    if (isa<FixedVectorType>(ScalarTy)) {
+      assert(SLPReVec && "Only supported by REVEC.");
+      // We don't need to insert elements one by one. Instead, we can insert the
+      // entire vector into the destination.
+      Cost = 0;
+      unsigned ScalarTyNumElements = getNumElements(ScalarTy);
+      for (unsigned I = 0, E = VL.size(); I != E; ++I)
----------------
alexey-bataev wrote:

```suggestion
      for (unsigned I : seq<unsigned>(VL.size()))
```

https://github.com/llvm/llvm-project/pull/109369