Add sse2 version for vp9_quantize_fp

vp9_quantize_fp is the quantization process used by rtc coding
mode. This commit adds a sse2 implementation of it. The
implementation is modified based on vp9_quantize_b_sse2. No speed
difference from ssse3 version.

Change-Id: I24949c5b27df160b4f35117d28858d269454e64a
Combine fdct8x8 and quantization process

This commit reworks the forward transform and quantization process
for 8x8 block coding. It combines the two operations in a single
function to save a store/load stage of the original transform
coefficients. Overall the speed -6 is slightly faster (around 1%
range). The compression performance of speed -6 is improved by
3.4%.

Change-Id: Id6628daef123f3e4649248735ec2ad7423629387
Merge "Add sse2 version for vp9_quantize_fp"
Merge "Combine fdct8x8 and quantization process"