FileCheck: fix matching of one check-prefix is a prefix of another

Summary:
Fix a case when "FileCheck --check-prefix=CHECK --check-prefix=CHECKER"
would silently ignore check-lines of the form:
  CHECKER: foo

Reviewers: dsanders

Reviewed By: dsanders

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2168

llvm-svn: 194577