Allow non-uniform above array in d45 predictor Neon impl

Allow non-uniform above array in d45 predictor Neon impl

The existing implementation doesn't appear to manifest as a failure in
any of the predictor or MD5 tests, but it does rely on the predictor
tests filling the second `bs` elements of the `above` input array with
copies of `above[bs - 1]` in order to match the C implementation.

This patch adjusts the Neon implementation to correctly match the C
implementation in the case where the elements of the `above` array all
differ.

Performance of the predictor is mostly unchanged, except for the 32x32
block size where it appears to have gotten about 40% faster when
compiled with clang-15.

Bug: webm:1797
Change-Id: Iaad58e77c5467307a3c80d6989b7cf2988e09311