[opt-viewer] Make filter_=None by default in get_remarks and gather_results

Right now, if you try to use optdiff.py on any opt records, it will fail because
its calls to gather_results weren't updated to support filtering.

Since filters are supposed to be optional, this makes them None by default in
get_remarks and in gather_results. This allows other tools that don't support
filtering to still use the functions as is.

Differential Revision: https://reviews.llvm.org/D59894

llvm-svn: 357106