Add support for undef values in ICE IR.

src/IceGlobalContext.h

 //===- subzero/src/IceGlobalContext.h - Global context defs -----*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file declares aspects of the compilation that persist across
@@ skipping 59 matching lines @@
 
   // Manage Constants.
   // getConstant*() functions are not const because they might add
   // something to the constant pool.
   Constant *getConstantInt(Type Ty, uint64_t ConstantInt64);
   Constant *getConstantFloat(float Value);
   Constant *getConstantDouble(double Value);
   // Returns a symbolic constant.
   Constant *getConstantSym(Type Ty, int64_t Offset, const IceString &Name = "",
                            bool SuppressMangling = false);
+  // Returns an undef.
+  Constant *getConstantUndef(Type Ty);
   // getConstantPool() returns a copy of the constant pool for
   // constants of a given type.
   ConstantList getConstantPool(Type Ty) const;
 
   // Allocate data of type T using the global allocator.
   template <typename T> T *allocate() { return Allocator.Allocate<T>(); }
 
 private:
   Ostream StrDump; // Stream for dumping / diagnostics
   Ostream StrEmit; // Stream for code emission
 
   llvm::BumpPtrAllocator Allocator;
   VerboseMask VMask;
   llvm::OwningPtr<class ConstantPool> ConstPool;
   const TargetArch Arch;
   const OptLevel Opt;
   const IceString TestPrefix;
   bool HasEmittedFirstMethod;
   GlobalContext(const GlobalContext &) LLVM_DELETED_FUNCTION;
   GlobalContext &operator=(const GlobalContext &) LLVM_DELETED_FUNCTION;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEGLOBALCONTEXT_H