pixelharness: only test partition sizes plausibly used by the encoder
pixel: only compile partition sizes that are used by the encoder
pixel: eliminate width 12 SAD code for height != 16
pixel: eliminate width 24 SAD code for height != 32
pixel: remove sad_*_4<> intrinsic functions, they are covered by assembly

x264 assembly code covers 4x4, 4x8, and 4x16 and those are the only 4-width
partitions used by x265.
pixel: simplify sad_64 primitives to a single loop

64 width blocks will only be height 16, 32, 48, or 64.  Having an 8-row loop
is just fine
pixel: simplify sad_32 primitives to a single loop

32 width blocks will only be height 8, 16, 24, or 32.  Having an 8-row loop
is just fine
pixel: eliminate width 48 SAD code for height != 64
pixel: limit sad_8 routines to just height 32

8x4, 8x8, and 8x16 are handled by x264 assembly, only 8x32 remains for the
intrinsic function to cover