arm32: loopfilter: NEON implementation of loopfilter for 16 bpc
arm32: loopfilter: NEON implementation of loopfilter for 16 bpc

This operates on 4 pixels as a time, while the arm64 version
operated on 8 pixels at a time.

As the registers only fit one single 4 pixel wide slice (with one
single set of input parameters and mask bits), the high level
logic for calculating those input parameters is done with GPRs
and scalar instructions instead of SIMD as in the other implementations.

xiph/rav1e,xiph/rav1e