utils/update_mir_test_checks.py: support UTC_ARGS

As a reminder, UTC_ARGS is used by lit test cases to specify which
arguments need to be passed to update_XXXX_test_checks.py to be
auto-updated properly.

The support is achieved by relying on common.itertests, which is what
other test
updaters use to iterate over test files.

This commit also changes how the --llc-binary option is saved in args.
It used to be saved as "llc", but it is here changed to the standard
"llc_binary" to make use of an existing ignore mechanism for specific
arguments. Without that change, the option would not be ignored and
would appear in UTC_ARGS. This would be different from what e.g.
update_llc_test_checks does. As update_mir_test_checks.py now supports
UTC_ARGS, it became important to ensure the option is ignored.

Differential Revision: https://reviews.llvm.org/D135580