Refactor standard bitdepth Neon convolution functions

Refactor standard bitdepth Neon convolution functions

1) Use #define constant instead of magic numbers for right shifts.
2) Move saturating narrow into helper functions that return 4-element
   result vectors.
3) Use mem_neon.h helpers for load/store sequences in Armv8.0 paths.
4) Tidy up: assert conditions and some longer variable names.
5) Prefer != 0 to > 0 where possible for loop termination conditions.

Change-Id: Idfcac43ca38faf729dca07b8cc8f7f45ad264d24