[LoopAccesses] Make raw_string_ostream local in VectorizationReport

[anemet]

Summary

Since VectorizationReport will be part of the result of the analysis it
will be stored in a container. However, one of its members is a
raw_string_ostream which cannot be copy-constructed.

This makes the raw_string_ostream local to the << operator.

This is part of the patchset that converts LoopAccessAnalysis into an
actual analysis pass.

[hfinkel]

Indeed; we normally use these as temporaries. LGTM.

[anemet]

r229620

include/llvm/Analysis/LoopAccessAnalysis.h

 class ScalarEvolution;
 class Loop;
 class SCEV;
 
 /// Optimization analysis message produced during vectorization. Messages inform
 /// the user why vectorization did not occur.
 class VectorizationReport {
   std::string Message;
-  raw_string_ostream Out;
   Instruction *Instr;
 
 public:
-  VectorizationReport(Instruction *I = nullptr) : Out(Message), Instr(I) {
-    Out << "loop not vectorized: ";
-  }
+  VectorizationReport(Instruction *I = nullptr)
+      : Message("loop not vectorized: "), Instr(I) {}
 
   template <typename A> VectorizationReport &operator<<(const A &Value) {
+    raw_string_ostream Out(Message);
     Out << Value;
     return *this;
   }
 
   Instruction *getInstr() { return Instr; }
 
-  std::string &str() { return Out.str(); }
-  operator Twine() { return Out.str(); }
+  std::string &str() { return Message; }
+  operator Twine() { return Message; }
 
   /// \brief Emit an analysis note with the debug location from the instruction
   /// in \p Message if available.  Otherwise use the location of \p TheLoop.
   static void emitAnalysis(VectorizationReport &Message,
                            const Function *TheFunction,
                            const Loop *TheLoop);
 };