Optimize Neon implementation of high bitdepth avg SAD functions

Optimizations take a similar form to those implemented for standard
bitdepth averaging SAD:

- Use ABD, UADALP instead of ABAL, ABAL2 (double the throughput on
  modern out-of-order Arm-designed cores.)
- Use more accumulator registers to make better use of Neon pipeline
  resources on Arm CPUs that have four Neon pipes.

Change-Id: I75c5f09948f6bf17200f82e00e7a827a80451108
1 file changed