8x8 transform support in splitmv.

For splitmv, where partitioning is 8x16, 16x8 or 8x8, this patch
uses the 8x8 transform (instead of the 4x4) if txfm_mode is
ALLOW_8X8 or ALLOW_16X16. For TX_MODE_SELECT, splitmv can indicate
which of the 2 transform sizes (4x4 or 8x8) it wants to use.

Gains (with hybridtx4x4/8x8/16x16 and tx_select experiments
enabled) on derf: +0.9%, HD: +0.4%, STD/HD: +0.8% (SSIM or overall
PSNR, both metrics show similar improvements).

Change-Id: Ide954b000b415548ed92a7ac78e24f36e60fcf06