[llvm-exegesis][NFC] Add missing header guard + cosmetics.

tools/llvm-exegesis/lib/RegisterValue.h

 //===-- RegisterValue.h -----------------------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
 ///
 /// Defines a Target independent value for a Register. This is useful to explore
 /// the influence of the instruction input values on its execution time.
 ///
 //===----------------------------------------------------------------------===//
 
+#ifndef LLVM_TOOLS_LLVM_EXEGESIS_REGISTERVALUE_H
+#define LLVM_TOOLS_LLVM_EXEGESIS_REGISTERVALUE_H
+
 #include <llvm/ADT/APFloat.h>
 #include <llvm/ADT/APInt.h>
 
 namespace llvm {
 namespace exegesis {
 
 // A simple object storing the value for a particular register.
 struct RegisterValue {
+  static RegisterValue zero(unsigned Reg) { return {Reg, llvm::APInt()}; }
   unsigned Register;
   llvm::APInt Value;
 };
 
 enum class PredefinedValues {
   POS_ZERO,       // Positive zero
   NEG_ZERO,       // Negative zero
   ONE,            // 1.0
   TWO,            // 2.0
   INF,            // Infinity
   QNAN,           // Quiet NaN
   ULP,            // One Unit in the last place
   SMALLEST = ULP, // The minimum subnormal number
   SMALLEST_NORM,  // The minimum normal number
   LARGEST,        // The maximum normal number
   ONE_PLUS_ULP,   // The value just after 1.0
 };
 
 llvm::APInt bitcastFloatValue(const llvm::fltSemantics &FltSemantics,
                               PredefinedValues Value);
 
 } // namespace exegesis
 } // namespace llvm
+
+#endif // LLVM_TOOLS_LLVM_EXEGESIS_REGISTERVALUE_H

tools/llvm-exegesis/lib/SnippetGenerator.cpp

const auto GetOpReg = [&IT](const Operand &Op) -> unsigned {
         return IT.getValueFor(Op).getReg();
       return 0;
     };
     // Collect used registers that have never been def'ed.
     for (const Operand &Op : IT.Instr.Operands) {
       if (Op.isUse()) {
         const unsigned Reg = GetOpReg(Op);
         if (Reg > 0 && !DefinedRegs.test(Reg)) {
-          RIV.push_back(RegisterValue{Reg, llvm::APInt()});
+          RIV.push_back(RegisterValue::zero(Reg));
           DefinedRegs.set(Reg);
         }
       }
     }
     // Mark defs as having been def'ed.
     for (const Operand &Op : IT.Instr.Operands) {
       if (Op.isDef()) {
         const unsigned Reg = GetOpReg(Op);