[llvm] [LV] Factor costInterleaveGatherScatter (NFC) (PR #215857)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -4416,50 +4485,13 @@ void LoopVectorizationCostModel::setCostBasedWideningDecision(ElementCount VF) {
       if (!Ptr)
         continue;
 
+      // Choose between Interleaving, Gather/Scatter or Scalarization.
+      auto [Decision, Cost] = costInterleaveGatherScatter(&I, VF);
----------------
artagnon wrote:

A bit confused by this: isn't this in the diff, after the guarded-continue?

```diff
-      // Choose between Interleaving, Gather/Scatter or Scalarization.
-      InstructionCost InterleaveCost = InstructionCost::getInvalid();
-      unsigned NumAccesses = 1;
-      if (isAccessInterleaved(&I)) {
-        const auto *Group = getInterleavedAccessGroup(&I);
-        assert(Group && "Fail to get an interleaved access group.");
-
-        // Make one decision for the whole group.
-        if (getWideningDecision(&I, VF) != CM_Unknown)
+      // Make one decision for the whole interleave group.
+      if (isAccessInterleaved(&I) && getWideningDecision(&I, VF) != CM_Unknown)
         continue;

-        NumAccesses = Group->getNumMembers();
-        if (interleavedAccessCanBeWidened(&I, VF))
-          InterleaveCost = getInterleaveGroupCost(&I, VF);
-      }
-
-      InstructionCost GatherScatterCost =
-          isLegalGatherOrScatter(&I, VF)
-              ? getGatherScatterCost(&I, VF) * NumAccesses
-              : InstructionCost::getInvalid();
-
-      InstructionCost ScalarizationCost =
-          getMemInstScalarizationCost(&I, VF) * NumAccesses;
-
-      // Choose better solution for the current VF,
-      // write down this decision and use it during vectorization.
-      InstructionCost Cost;
-      InstWidening Decision;
-      if (InterleaveCost <= GatherScatterCost &&
-          InterleaveCost < ScalarizationCost) {
-        Decision = CM_Interleave;
-        Cost = InterleaveCost;
-      } else if (GatherScatterCost < ScalarizationCost) {
-        Decision = CM_GatherScatter;
-        Cost = GatherScatterCost;
-      } else {
-        Decision = CM_Scalarize;
-        Cost = ScalarizationCost;
-      }
``

How else would we initialize Decision and Cost for use later?

https://github.com/llvm/llvm-project/pull/215857