[LV] Set correct costs for interleave group members.

This ensures each scalarized member has an accurate cost, matching the
cost it would have if it would not have been considered for an
interleave group.

(cherry picked from commit 49605a4)

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -5695,9 +5695,18 @@ void LoopVectorizationCostModel::setCostBasedWideningDecision(ElementCount VF) {
       // If the instructions belongs to an interleave group, the whole group
       // receives the same decision. The whole group receives the cost, but
       // the cost will actually be assigned to one instruction.
-      if (const auto *Group = getInterleavedAccessGroup(&I))
-        setWideningDecision(Group, VF, Decision, Cost);
-      else
+      if (const auto *Group = getInterleavedAccessGroup(&I)) {
+        if (Decision == CM_Scalarize) {
+          for (unsigned Idx = 0; Idx < Group->getFactor(); ++Idx) {
+            if (auto *I = Group->getMember(Idx)) {
+              setWideningDecision(I, VF, Decision,
+                                  getMemInstScalarizationCost(I, VF));
+            }
+          }
+        } else {
+          setWideningDecision(Group, VF, Decision, Cost);
+        }
+      } else
         setWideningDecision(&I, VF, Decision, Cost);
     }
   }