Make PNGImageDecoder::rowAvailable auto-vectorizable
https://bugs.webkit.org/show_bug.cgi?id=116151

Reviewed by Benjamin Poulain.

Changed the main loops under PNGImageDecoder::rowAvailable so that they
avoid branches and non-sequential table look ups.

Together with automatic vectorization by the compiler this provides around
4x speed-up with AVX or 2x speed-up on generic x64. Shaving off 12-40% on
PNG decoding in general.

* platform/graphics/Color.cpp:
(WebCore::premultipliedARGBFromColor):
* platform/graphics/Color.h:
(WebCore::fastDivideBy255):
* platform/graphics/filters/FEBlend.cpp:
* platform/image-decoders/png/PNGImageDecoder.cpp:
(WebCore::setPixelRGB):
(WebCore::setPixelRGBA):
(WebCore::setPixelRGBA_Premultiplied):
(WebCore::PNGImageDecoder::rowAvailable):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150252 268f45cc-cd09-0410-ab3c-d52691b4dbfc