Revised SSE2 color conversion
Revised SSE2 color conversion

I discovered there's average and shuffle instructions which simplify the
calculations of UV components. Also the extract instruction was very useful.

In terms of the algorithm I rearranged the memory reads to be sequential.

Overall this gives about 10% improvement on 64-bits machine.

On 32-bits machine this gives 20% improvement. If compiled as 32-bits code this is
quite slow compared to 64-bits, about 2x slow down. So working on MMX code will
be a direction for improvement.

BUG=None
TEST=I tried it, it looks right.

Review URL: http://codereview.chromium.org/6488014

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@74827 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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