[DeadStoreElimination] Use SmallSetVector (NFC) (#79410)

The use of SmallSetVector saves 0.58% of heap allocations during the
compilation of a large preprocessed file, namely X86ISelLowering.cpp,
for the X86 target.  During the experiment, the final size of ToCheck
was 8 or less 88% of the time.

Author: kazutakahirata

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -2096,7 +2096,7 @@ static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
     unsigned WalkerStepLimit = MemorySSAUpwardsStepLimit;
     unsigned PartialLimit = MemorySSAPartialStoreLimit;
     // Worklist of MemoryAccesses that may be killed by KillingDef.
-    SetVector<MemoryAccess *> ToCheck;
+    SmallSetVector<MemoryAccess *, 8> ToCheck;
     ToCheck.insert(KillingDef->getDefiningAccess());
 
     bool Shortend = false;