[FileCheck] Fix a bug that cause FileCheck to misidentify check-prefix

FileCheck should check to make sure the prefix was found, and not a word
containing it (e.g -check-prefix=BASEREL shouldn't match NOBASEREL).

Patch by Ron Ofir.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188221 91177308-0d34-0410-b5e6-96231b3b80d8