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