asm: active x264 ssim code

Side effect:
Remove ssim_t to avoid conflict with x264 asm code
we use int64_t when HIGH_BIT_DEPTH enable, but x264 always 'int'
fix: in 14bpp mode, maximum shift is 10
asm: removed unused code in pixel_var module
asm: code for pixel_sse_sp_16xN
asm: assembly code for intra_pred_planar[8x8]
asm: Correct number of xmm registers for weight_sp routine.
asm: assembly code for dct4x4
Adding constant table used for dct4
Adding dct8.asm and dct8.h to CMakeLists
Enable the new dct4 asm routine.
Merge
vec: remove dct4 intrinsic primitive
vec: drop dequant_normal intrinsic primitive, we have asm
encoder: return pic.sliceType = X265_TYPE_IDR if keyframe
vec: drop intra_pred_planar 4x4 and 8x8 intrinsic primitives, we have asm