Add an option to run the inst-count pass after performing optimizations.

This will enable via the -print-stats function the ability to quickly
find out the final count of various forms of instructions. My intention
is to use this to count retains and releases.

Swift SVN r18946

lib/Frontend/CompilerInvocation.cpp

@@ -776,6 +776,7 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   Opts.PrintAll |= Args.hasArg(OPT_sil_print_all);
   Opts.TimeTransforms |= Args.hasArg(OPT_sil_time_transforms);
   Opts.DebugSerialization |= Args.hasArg(OPT_sil_debug_serialization);
+  Opts.PrintInstCounts |= Args.hasArg(OPT_print_inst_counts);
 
   return false;
 }