[utils] Improve extraction of check prefixes from RUN lines

Correct handling of the following FileCheck options is implemented in
update_llc_test_checks.py and update_test_checks.py scripts:

1) -check-prefix (with a single dash)
2) -check-prefixes (with multiple prefixes)

Differential Revision: https://reviews.llvm.org/D28572

llvm-svn: 292008