asm: Minor pixel_ssim_end4 improvements

Reduce the number of vector registers used from 7 to 5.
Eliminate some moves in the AVX implementation.
Avoid bypass delays for transitioning between int and
float domains.

Ported from Henrik Gramner's recent commit to x264
x86inc: Make INIT_CPUFLAGS support an arbitrary number of cpuflags

Ported from Henrik Gramner's recent commit to x264
framefilter: move SAO init logic into the frame filter
sao: don't pass member variables to functions