x86_64 RDO cabac_block_residual assembly

~10-20% faster than C.
Fix some store forwarding stalls
There's quite a few others, but most of them don't help to fix or there's no
easy way to avoid them.
Eliminate some branchiness in ME/analysis

Faster, fewer branch mispredictions.
quant_4x4x4: quant one 8x8 block at a time

This reduces overhead and lets us use less branchy code for zigzag, dequant,
decimate, and so on.
Reorganize and optimize a lot of macroblock_encode using this new function.
~1-2% faster overall.

Includes NEON and x86 versions of the new function.
Using larger merged functions like this will also make wider SIMD, like
AVX2, more effective.