Test this differently: I saw this test fail

because opt exited while llvm-as was still
writing to the pipe, causing it to get a
SIGPIPE.  It seems best to change things to
avoid the race altogether.

llvm-svn: 54138

llvm/test/Other/invalid-commandline-option.ll

@@ -1,3 +1,3 @@
-; RUN: llvm-as < /dev/null | not opt --foo >& /dev/null
+; RUN: not opt --foo |& grep {Unknown command line argument}
 
 ; there is no --foo