[IR] Add DenseMapInfo<CallSite>.

[jlebar]

Summary

A CallSite is basically an Instruction*, and you can put Instruction*s
into DenseMaps, so you should be able to do the same with CallSites.

This is used in a later patch.

llvm/include/llvm/IR/CallSite.h

public:
 
   bool operator==(const CallSite &CS) const { return I == CS.I; }
   bool operator!=(const CallSite &CS) const { return I != CS.I; }
   bool operator<(const CallSite &CS) const {
     return getInstruction() < CS.getInstruction();
   }
 
 private:
+  friend struct DenseMapInfo<CallSite>;
   User::op_iterator getCallee() const;
 };
 
+template <> struct DenseMapInfo<CallSite> {
+  using BaseInfo = llvm::DenseMapInfo<decltype(CallSite::I)>;
+
+  static CallSite getEmptyKey() {
+    CallSite CS;
+    CS.I = BaseInfo::getEmptyKey();
+    return CS;
+  }
+  static CallSite getTombstoneKey() {
+    CallSite CS;
+    CS.I = BaseInfo::getTombstoneKey();
+    return CS;
+  }
+  static unsigned getHashValue(const CallSite &CS) {
+    return BaseInfo::getHashValue(CS.I);
+  }
+  static bool isEqual(const CallSite &LHS, const CallSite &RHS) {
+    return LHS == RHS;
+  }
+};
+
 /// ImmutableCallSite - establish a view to a call site for examination
 class ImmutableCallSite : public CallSiteBase<> {
 public:
   ImmutableCallSite() {}
   ImmutableCallSite(const CallInst *CI) : CallSiteBase(CI) {}
   ImmutableCallSite(const InvokeInst *II) : CallSiteBase(II) {}
   explicit ImmutableCallSite(const Instruction *II) : CallSiteBase(II) {}
   explicit ImmutableCallSite(const Value *V) : CallSiteBase(V) {}
   ImmutableCallSite(CallSite CS) : CallSiteBase(CS.getInstruction()) {}
 };
 
 } // End llvm namespace
 
 #endif