[utils] Stricter checking from update_mca_test_checks.py

If any prefixes have been specified on the RUN lines that do not end up
ever actually getting printed, raise an Error. This is either an
indication that the run lines just need cleaning up, or that something
is more fundamentally wrong with the test.

Also raise an Error if there are any blocks which cannot be checked
because they are not uniquely covered by a prefix.

Fixed up a couple of tests where the extra checking flagged up issues.

Differential Revision: https://reviews.llvm.org/D48276

llvm-svn: 343332