[llvm] [SLP]: Introduce and use getDataFlowCost (PR #112999)

[Matt Arsenault via llvm-commits]

================
@@ -10241,6 +10287,31 @@ InstructionCost BoUpSLP::getTreeCost(ArrayRef<Value *> VectorizedVals) {
 
     InstructionCost C = getEntryCost(&TE, VectorizedVals, CheckedExtracts);
     Cost += C;
+
+    // Calculate the cost difference of propagating a vector vs series of scalars
+    // across blocks. This may be nonzero in the case of illegal vectors.
+    Instruction *VL0 = TE.getMainOp();
+    bool IsAPhi = VL0 && isa<PHINode>(VL0);
+    bool HasNextEntry = VL0 && ((I + 1) < VectorizableTree.size());
+    bool LiveThru = false;
+    if (HasNextEntry) {
+      Instruction *VL1 = VectorizableTree[I + 1]->getMainOp();
+      LiveThru = VL1 && (VL0->getParent() != VL1->getParent());
+    }
+    if (IsAPhi || LiveThru) {
+      VectorType *VTy = dyn_cast<VectorType>(VL0->getType());
+      Type *ScalarTy = VTy ? VTy->getElementType() : VL0->getType();
+      if (ScalarTy && isValidElementType(ScalarTy)) {
+        InstructionCost ScalarDFlow =
+            TTI->getDataFlowCost(ScalarTy,
+                                 /*IsCallingConv*/ false) *
----------------
arsenm wrote:

```suggestion
                                 /*IsCallingConv=*/ false) *
```

https://github.com/llvm/llvm-project/pull/112999