[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292008 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed