h264pred: added AVX2 implementation for tm_vp8 16x16.

checkasm --bench results with 5000 runs

pred16x16_tm_vp8_c: 302.8
pred16x16_tm_vp8_mmx: 101.4
pred16x16_tm_vp8_mmxext: 95.5
pred16x16_tm_vp8_sse2: 95.1
pred16x16_tm_vp8_avx2: 38.2

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
