[JumpThreading] Use SmallPtrSet (NFC) (llvm#95674)

The use of SmallPtrSet here saves 0.66% of heap allocations during
the compilation of a large preprocessed file, namely
X86ISelLowering.cpp, for the X86 target.

Author: kazutakahirata

llvm/include/llvm/Transforms/Scalar/JumpThreading.h

@@ -130,13 +130,13 @@ class JumpThreadingPass : public PassInfoMixin<JumpThreadingPass> {
   bool computeValueKnownInPredecessorsImpl(
       Value *V, BasicBlock *BB, jumpthreading::PredValueInfo &Result,
       jumpthreading::ConstantPreference Preference,
-      DenseSet<Value *> &RecursionSet, Instruction *CxtI = nullptr);
+      SmallPtrSet<Value *, 4> &RecursionSet, Instruction *CxtI = nullptr);
   bool
   computeValueKnownInPredecessors(Value *V, BasicBlock *BB,
                                   jumpthreading::PredValueInfo &Result,
                                   jumpthreading::ConstantPreference Preference,
                                   Instruction *CxtI = nullptr) {
-    DenseSet<Value *> RecursionSet;
+    SmallPtrSet<Value *, 4> RecursionSet;
     return computeValueKnownInPredecessorsImpl(V, BB, Result, Preference,
                                                RecursionSet, CxtI);
   }

llvm/lib/Transforms/Scalar/JumpThreading.cpp

@@ -558,7 +558,7 @@ static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
 /// This returns true if there were any known values.
 bool JumpThreadingPass::computeValueKnownInPredecessorsImpl(
     Value *V, BasicBlock *BB, PredValueInfo &Result,
-    ConstantPreference Preference, DenseSet<Value *> &RecursionSet,
+    ConstantPreference Preference, SmallPtrSet<Value *, 4> &RecursionSet,
     Instruction *CxtI) {
   const DataLayout &DL = BB->getModule()->getDataLayout();