x86_64 RDO cabac_block_residual assembly
~10-20% faster than C
FIXME: currently broken with no-asm
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_search_ref/refine_subpel
Faster, fewer branch predictions.
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.
~17% faster macroblock_encode with 4x4dct.

Using larger merged functions like this will also make wider SIMD, like
AVX2, more effective.