[MC] EmitNop: Make an assertion more useful

[davide]

Summary

NumBytes is an unsigned so this will never fire. I assume this might be what you wanted to do originally.

[sanjoy]

Gah, made a C++ 101 mistake. :|

Thanks for fixing!

lib/Target/X86/X86MCInstLower.cpp

 /// \brief Emit the optimal amount of multi-byte nops on X86.
 static void EmitNops(MCStreamer &OS, unsigned NumBytes, bool Is64Bit,
                      const MCSubtargetInfo &STI) {
+  unsigned NopsToEmit = NumBytes;
   while (NumBytes) {
     NumBytes -= EmitNop(OS, NumBytes, Is64Bit, STI);
-    assert(NumBytes >= 0 && "Emitted more than I asked for!");
+    assert(NopsToEmit >= NumBytes && "Emitted more than I asked for!");
   }
 }