Revert float xfermodes back to Sk4f (from Sk8f).

Revert float xfermodes back to Sk4f (from Sk8f).

Generally this was a performance win, even on devices without AVX due
to unrolling, but on ARM+NEON it looks like that unrolling hurt a bit.

    while (...) { blend a pixel }
~~~>
    while (...) { blend two pixels }
    if (n % 2) { blend last pixel }


BUG=chromium:555278
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Committed: https://skia.googlesource.com/skia/+/29d60e5ab594b39d1f533bff090877b1bb821e06

[mtklein_C, 2015-11-19 16:54:00]

Description was changed from

==========
Revert float xfermodes back to Sk4f (from Sk8f).

Generally this was a performance win, but on ARM+NEON it looks like the
implicit unrolling from
    while (...) { blend a pixel }
to
    while (...) { blend two pixels }
    if (n % 2) { blend last pixel }
slowed things down.

BUG=chromium:555278
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot
==========

to

==========
Revert float xfermodes back to Sk4f (from Sk8f).

Generally this was a performance win, even on devices without AVX due
to unrolling, but on ARM+NEON it looks like that unrolling hurt a bit.

    while (...) { blend a pixel }
~~~>
    while (...) { blend two pixels }
    if (n % 2) { blend last pixel }


BUG=chromium:555278
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot
==========

[mtklein_C, 2015-11-19 16:54:38]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-11-19 16:54:48]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1465483002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1465483002/1

[mtklein, 2015-11-19 17:03:29]

The CQ bit was unchecked by mtklein@google.com

[mtklein, 2015-11-19 17:03:31]

mtklein@google.com changed reviewers:
+ mtklein@google.com

[mtklein, 2015-11-19 17:03:31]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-11-19 17:03:31]

lgtm

[commit-bot: I haz the power, 2015-11-19 17:03:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1465483002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1465483002/1

[commit-bot: I haz the power, 2015-11-19 17:10:35]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/29d60e5ab594b39d1f533bff090877b1bb821e06