New testcase (compare2.c) for bogus -Wsign-compare warnings

[Jeffrey A Law]

  In message < 200004061646.MAA27374@caip.rutgers.edu >you write:
  > Admittedly, I'm not volunteering to supply a patch to gcc to allow
  > this.  But I'm hoping a testcase might inspire someone. :-)
  > 
  > Okay to install as gcc.dg/compare2.c ?
Yes, the test is fine.
jeff