Turn on concave analytic AA

Turn on concave analytic AA

This will allow Skia trybots to exercise analytic AA. But there's still
a guard flag in Chromium that prevents layout tests failure.

Additionally, we
1. fixed nagative shift problem
2. relax the ASSERT when slope is too large: If slope is large, the accuracy of the slope is limited due
to conversion to SkFDot6 and division. Hence we have to relax the constraint.
3. handle the special case where dx != 0 while slope == 0 because of very large dy and limited precision.

BUG=skia:

Change-Id: Ice70164f3f396f0db3896bedc7b96fbd613078dc
Reviewed-on: https://skia-review.googlesource.com/7120
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>