[llvm] [SLP][NFC] Redesign schedule bundle, separate from schedule data, NFC (PR #131625)

[Gaëtan Bossu via llvm-commits]

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@@ -3781,22 +3782,23 @@ class BoUpSLP {
         }
       }
       // Update the scheduler bundle to point to this TreeEntry.
-      ScheduleData *BundleMember = *Bundle;
-      assert((BundleMember || isa<PHINode>(S.getMainOp()) ||
+      assert((!Bundle.getBundle().empty() || isa<PHINode>(S.getMainOp()) ||
               isVectorLikeInstWithConstOps(S.getMainOp()) ||
               doesNotNeedToSchedule(VL)) &&
              "Bundle and VL out of sync");
-      if (BundleMember) {
+      if (!Bundle.getBundle().empty()) {
+        auto *BundleMember = Bundle.getBundle().begin();
         for (Value *V : VL) {
           if (doesNotNeedToBeScheduled(V))
             continue;
-          if (!BundleMember)
+          if (BundleMember == Bundle.getBundle().end())
             continue;
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gbossu wrote:

It's not clear to me what exactly is being processed in the loop. It seems it is only incrementing an iterator, and the latter is only used in an `assert`. Maybe `doesNotNeedToBeScheduled()` does more than its name suggests? I'm probably missing something, sorry 😅 

https://github.com/llvm/llvm-project/pull/131625