[Vectorize] Fix an 'unused variable' warning

This patch fixes:

  llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:8148:18: error:
  unused variable 'SDTE' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -8146,6 +8146,7 @@ void BoUpSLP::scheduleBlock(BlockScheduling *BS) {
        I = I->getNextNode()) {
     BS->doForAllOpcodes(I, [this, &Idx, &NumToSchedule, BS](ScheduleData *SD) {
       TreeEntry *SDTE = getTreeEntry(SD->Inst);
+      (void)SDTE;
       assert((isVectorLikeInstWithConstOps(SD->Inst) ||
               SD->isPartOfBundle() ==
                   (SDTE && !doesNotNeedToSchedule(SDTE->Scalars))) &&