Avoid LD2/ST2 instructions in highbd v predictors in Neon

Avoid LD2/ST2 instructions in highbd v predictors in Neon

The interleaving load/store instructions (LD2/LD3/LD4 and ST2/ST3/ST4)
are useful if we are dealing with interleaved data (e.g. real/imag
components of complex numbers), but for simply loading or storing larger
quantities of data it is preferable to simply use the normal load/store
instructions.

This patch replaces such occurrences in the two larger block sizes:
vpx_highbd_v_predictor_16x16_neon and vpx_highbd_v_predictor_32x32_neon.

Change-Id: Ie4ffa298a2466ceaf893566fd0aefe3f66f439e4