Make EmitFunctionHeader virtual

[arsenm]

Summary

This is to fix problems introduced by r232481. For HSAIL,
this function does essentially nothing desirable, and
injects unwanted / incorrect stuff before the function.
The only thing it really needs to do is call EmitFunctionEntryLabel
in this case.

[• rafael]

LGTM with the condition that the HSAIL backend will show up soon :-)

[arsenm]

r238222.

HSAIL is already posted to http://reviews.llvm.org/D9751!

include/llvm/CodeGen/AsmPrinter.h

 private:
   /// Private state for PrintSpecial()
   // Assign a unique ID to this machine instruction.
   mutable const MachineInstr *LastMI;
   mutable unsigned LastFn;
   mutable unsigned Counter;
 
   /// This method emits the header for the current function.
-  void EmitFunctionHeader();
+  virtual void EmitFunctionHeader();
 
   /// Emit a blob of inline asm to the output streamer.
   void
   EmitInlineAsm(StringRef Str, const MCSubtargetInfo &STI,
                 const MDNode *LocMDNode = nullptr,
                 InlineAsm::AsmDialect AsmDialect = InlineAsm::AD_ATT) const;
 
   /// This method formats and emits the specified machine instruction that is an