arm: use available neon functions for intra_sa8d/sad/satd_x3

4% faster on main/medium, 15% faster on baseline/superfast on a cortex-a9.
Macroblock tree overhaul/optimization

Move the second core part of macroblock tree into an assembly function;
SIMD-optimize roughly half of it (for x86). Roughly ~25-65% faster mbtree,
depending on content.

Slightly change how mbtree handles the tradeoff between range and precision
for propagation.

Overall a slight (but mostly negligible) effect on SSIM and ~2% faster.