[SLP][NFC] Fix condition that was supposed to save a bit of compile time.

[vdmitrie]

Summary

It was found by chance revealing discrepancy between comment, few lines above,
the condition and how re-ordering of instruction is done inside the if statement it guards.
The condition was always evaluated to true.

[ABataev]

LG

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

while (!ReadyInsts.empty()) {
     ScheduleData *picked = *ReadyInsts.begin();
     ReadyInsts.erase(ReadyInsts.begin());
 
     // Move the scheduled instruction(s) to their dedicated places, if not
     // there yet.
     ScheduleData *BundleMember = picked;
     while (BundleMember) {
       Instruction *pickedInst = BundleMember->Inst;
-      if (LastScheduledInst->getNextNode() != pickedInst) {
+      if (pickedInst->getNextNode() != LastScheduledInst) {
         BS->BB->getInstList().remove(pickedInst);
         BS->BB->getInstList().insert(LastScheduledInst->getIterator(),
                                      pickedInst);
       }
       LastScheduledInst = pickedInst;
       BundleMember = BundleMember->NextInBundle;
     }