[LLVMContextImpl] Separate out integer constant ones

Very small compile time improvement:
https://llvm-compile-time-tracker.com/compare.php?from=6a7a8907e8334eaf551742148079c628f78e6ed7&to=454d1181fbdb9121f0c7a3ecf526520db32ab420&stat=instructions:u

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D144746

llvm/lib/IR/Constants.cpp

@@ -875,8 +875,9 @@ 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 =
-      V.isZero() ? pImpl->IntZeroConstants[V.getBitWidth()]
-                 : pImpl->IntConstants[V];
+      V.isZero()  ? pImpl->IntZeroConstants[V.getBitWidth()]
+      : V.isOne() ? pImpl->IntOneConstants[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());

llvm/lib/IR/LLVMContextImpl.cpp

@@ -117,6 +117,7 @@ LLVMContextImpl::~LLVMContextImpl() {
   UVConstants.clear();
   PVConstants.clear();
   IntZeroConstants.clear();
+  IntOneConstants.clear();
   IntConstants.clear();
   FPConstants.clear();
   CDSConstants.clear();

llvm/lib/IR/LLVMContextImpl.h

@@ -1447,6 +1447,7 @@ class LLVMContextImpl {
   DenseMap<const Value *, ValueName *> ValueNames;
 
   DenseMap<unsigned, std::unique_ptr<ConstantInt>> IntZeroConstants;
+  DenseMap<unsigned, std::unique_ptr<ConstantInt>> IntOneConstants;
   DenseMap<APInt, std::unique_ptr<ConstantInt>, DenseMapAPIntKeyInfo>
       IntConstants;