fixed an overflow in ssim calculation

This commit fixed an overflow in ssim calculation, added register
save and restore to make sure assembly code working for x64 platform.
It also changed the sampling points to every 4x4 instead of 8x8 and
adjusted the constants in SSIM calculation to match the scale of
previous VPXSSIM.

Change-Id: Ia4dbb8c69eac55812f4662c88ab4653b6720537b