[dfsan] Add -dfsan-fast-8-labels flag

This is only adding support to the dfsan instrumentation pass but not
to the runtime.

Added more RUN lines for testing: for each instrumentation test that
had a -dfsan-fast-16-labels invocation, a new invocation was added
using fast8.

Reviewed By: stephan.yichao.zhao

Differential Revision: https://reviews.llvm.org/D98734