Teach ISel not to optimize 'optnone' functions (revised).

Improvements over r195317:
- Set/restore EnableFastISel flag instead of just running FastISel within
  SelectAllBasicBlocks; the flag is checked in various places, and
  FastISel won't run properly if those places don't do the right thing.
- Test looks for normal ISel versus FastISel behavior, and not
  something more subtle that doesn't work everywhere.

Based on work by Andrea Di Biagio.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195491 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetMachine.h

@@ -75,7 +75,8 @@ protected: // Can only create subclasses.
   std::string TargetFS;
 
   /// CodeGenInfo - Low level target information such as relocation model.
-  const MCCodeGenInfo *CodeGenInfo;
+  /// Non-const to allow resetting optimization level per-function.
+  MCCodeGenInfo *CodeGenInfo;
 
   /// AsmInfo - Contains target specific asm information.
   ///
@@ -213,6 +214,9 @@ public:
   /// Default, or Aggressive.
   CodeGenOpt::Level getOptLevel() const;
 
+  /// \brief Overrides the optimization level.
+  void setOptLevel(CodeGenOpt::Level Level) const;
+
   void setFastISel(bool Enable) { Options.EnableFastISel = Enable; }
 
   bool shouldPrintMachineCode() const { return Options.PrintMachineCode; }