the return of bit_clear

bit_clear is at least useful as a special case for select(),
which helps with code readability.

Add is_NaN() and use these all together in sweep gradient.

Change-Id: I57a54f8956f85e0db0662b33f8446b8dc7342d8d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/281685
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
7 files changed