Restore vp8_bilinear_predict4x4_neon

Restore vp8_bilinear_predict4x4_neon

This function was removed when clang started introducing alignment hints
which caused the 32 bit vld1_lane_u32/vst1_lane_u32 to fail:
https://llvm.org/bugs/show_bug.cgi?id=24421

The load has been rendered safe with an implementation ~indiscernible
performance-wise that uses _u8 and over-reads just a touch.

It is still ~5x faster than C in the unaligned case and doing both
filters.

BUG=webm:892
BUG=webm:1273

Change-Id: Icf7167189391b46202f47233bb585c24c42bcc36