Pad with a value which works both as a large unsigned value and a
negative signed value. This allows doing the max operation using
signed max, avoiding the conditional altogether.
Based on the same idea for x86 by Kyle Siefring.
Before: Cortex A53 A72 A73
cdef_filter_4x4_8bpc_neon: 645.5 401.9 422.5
cdef_filter_4x8_8bpc_neon: 1193.7 756.6 782.4
cdef_filter_8x8_8bpc_neon: 2162.4 1361.9 1375.6
After:
cdef_filter_4x4_8bpc_neon: 596.3 377.8 384.8
cdef_filter_4x8_8bpc_neon: 1097.4 705.5 707.1
cdef_filter_8x8_8bpc_neon: 1967.4 1232.3 1239.9