The denoiser function was modified to reduce the computational
complexity.

1. The denoiser c function modification:
The original implementation calculated pixel's filter_coefficient
based on the pixel value difference between current raw frame and last
denoised raw frame, and stored them in lookup tables. For each pixel c,
find its coefficient using
    filter_coefficient[c] = LUT[abs_diff[c]];
and then apply filtering operation for the pixel.

The denoising filter costed about 12% of encoding time when it was
turned on, and half of the time was spent on finding coefficients in
lookup tables. In order to simplify the process, a short cut was taken.
The pixel adjustments vs. pixel diff value were calculated ahead of time.
    adjustment = filtered_value - current_raw
               = (filter_coefficient * diff + 128) >> 8

The adjustment vs. diff curve becomes flat very quick when diff increases.
This allowed us to use only several levels to get a close approximation
of the curve. Following the denoiser algorithm, the adjustments are
further modified according to how big the motion magnitude is.

2. The sse2 function was rewritten.

This change made denoiser filter function 3x faster, and improved the
encoder performance by 7% ~ 10% with the denoiser on.

Change-Id: I93a4308963b8e80c7307f96ffa8b8c667425bf50