[frontend] Fix obvious typo.

Just found via inspection.

Author: gottesmm

lib/Frontend/CompilerInvocation.cpp

@@ -843,7 +843,7 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   // -Ounchecked might also set removal of runtime asserts (cond_fail).
   Opts.RemoveRuntimeAsserts |= Args.hasArg(OPT_RemoveRuntimeAsserts);
 
-  Opts.EnableARCOptimizations |= !Args.hasArg(OPT_disable_arc_opts);
+  Opts.EnableARCOptimizations &= !Args.hasArg(OPT_disable_arc_opts);
   Opts.EnableOSSAOptimizations &= !Args.hasArg(OPT_disable_ossa_opts);
   Opts.DisableSILPerfOptimizations |= Args.hasArg(OPT_disable_sil_perf_optzns);
   Opts.VerifyAll |= Args.hasArg(OPT_sil_verify_all);