[LLVMContextImpl] Separate out integer constant zeroes

[aeubanks]

Summary

Very small compile time improvement:
https://llvm-compile-time-tracker.com/compare.php?from=a628ca4925f7249b4fbd3e932c9627b12e2770dd&to=6a7a8907e8334eaf551742148079c628f78e6ed7&stat=instructions:u

[nikic]

LG

llvm/lib/IR/Constants.cpp

 Constant *ConstantInt::getBool(Type *Ty, bool V) {
   return V ? getTrue(Ty) : getFalse(Ty);
 }
 
 // Get a ConstantInt from an APInt.
 ConstantInt *ConstantInt::get(LLVMContext &Context, const APInt &V) {
   // get an existing value or the insertion position
   LLVMContextImpl *pImpl = Context.pImpl;
-  std::unique_ptr<ConstantInt> &Slot = pImpl->IntConstants[V];
+  std::unique_ptr<ConstantInt> &Slot =
+      V.isZero() ? pImpl->IntZeroConstants[V.getBitWidth()]
+                 : pImpl->IntConstants[V];
   if (!Slot) {
     // Get the corresponding integer type for the bit width of the value.
     IntegerType *ITy = IntegerType::get(Context, V.getBitWidth());
     Slot.reset(new ConstantInt(ITy, V));
   }
   assert(Slot->getType() == IntegerType::get(Context, V.getBitWidth()));
   return Slot.get();
 }

llvm/lib/IR/LLVMContextImpl.h

public:
   /// The specialized remark streamer used by LLVM's OptimizationRemarkEmitter.
   std::unique_ptr<LLVMRemarkStreamer> LLVMRS;
 
   LLVMContext::YieldCallbackTy YieldCallback = nullptr;
   void *YieldOpaqueHandle = nullptr;
 
   DenseMap<const Value *, ValueName *> ValueNames;
 
-  using IntMapTy =
-      DenseMap<APInt, std::unique_ptr<ConstantInt>, DenseMapAPIntKeyInfo>;
-  IntMapTy IntConstants;
-
-  using FPMapTy =
-      DenseMap<APFloat, std::unique_ptr<ConstantFP>, DenseMapAPFloatKeyInfo>;
-  FPMapTy FPConstants;
+  DenseMap<unsigned, std::unique_ptr<ConstantInt>> IntZeroConstants;
+  DenseMap<APInt, std::unique_ptr<ConstantInt>, DenseMapAPIntKeyInfo>
+      IntConstants;
+
+  DenseMap<APFloat, std::unique_ptr<ConstantFP>, DenseMapAPFloatKeyInfo>
+      FPConstants;
 
   FoldingSet<AttributeImpl> AttrsSet;
   FoldingSet<AttributeListImpl> AttrsLists;
   FoldingSet<AttributeSetNode> AttrsSetNodes;
 
   StringMap<MDString, BumpPtrAllocator> MDStringCache;
   DenseMap<Value *, ValueAsMetadata *> ValuesAsMetadata;
   DenseMap<Metadata *, MetadataAsValue *> MetadataAsValues;

llvm/lib/IR/LLVMContextImpl.cpp

#include "llvm/IR/Metadata.def"
   VectorConstants.freeConstants();
   InlineAsms.freeConstants();
 
   CAZConstants.clear();
   CPNConstants.clear();
   CTNConstants.clear();
   UVConstants.clear();
   PVConstants.clear();
+  IntZeroConstants.clear();
   IntConstants.clear();
   FPConstants.clear();
   CDSConstants.clear();
 
   // Destroy attribute node lists.
   for (FoldingSetIterator<AttributeSetNode> I = AttrsSetNodes.begin(),
          E = AttrsSetNodes.end(); I != E; ) {
     FoldingSetIterator<AttributeSetNode> Elem = I++;