Fix asm label testcase flaw

- Testcase attempts to (not) grep 'g0' in output to ensure asm symbol is
  properly renamed, but g0 is too generic and can be part of the
  module's path in LLVM IR output.
- Changed to grep '@g0', which is what the proper global symbol name
  would be if not using asm.

llvm-svn: 338895