Tighten up a test for -check-debugify, NFC

Use an -implicit-check-not to make sure an error which should not occur
in fact does not occur before the first CHECK line.

Suggested by Paul Robinson in post-commit feedback for r335897.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336123 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed