Refactor FourByteInterps. Add 64-bit Fast version. Add tests.

Refactor FourByteInterps.  Add 64-bit Fast version.  Add tests.

Effect on benches (only _fast_ should be affected, and only on 64-bit):

Desktop (64-bit)
four_byte_interp_slow_255 NONRENDERING c          7.80       7.84      -0.04    -0.5%
four_byte_interp_slow_256 NONRENDERING c          7.38       7.36      +0.02    +0.3%
four_byte_interp_fast_256 NONRENDERING c          4.86       4.38      +0.48    +9.9%
four_byte_interp_fast_255 NONRENDERING c          5.80       5.16      +0.64   +11.0%

N5 (32-bit)
four_byte_interp_slow_256 NONRENDERING c         22.22      22.66      -0.44    -2.0%
four_byte_interp_fast_255 NONRENDERING c         22.22      22.22      +0.00    +0.0%
four_byte_interp_fast_256 NONRENDERING c         18.81      18.81      +0.00    +0.0%
four_byte_interp_slow_255 NONRENDERING c         22.42      22.42      +0.00    +0.0%

BUG=
R=reed@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/100923003

git-svn-id: http://skia.googlecode.com/svn/trunk@12468 2bbb7eff-a529-9590-31e7-b0007b416f81