arm64: looprestoration16: Don't keep precalculated squares in box3/5_h
arm64: looprestoration16: Don't keep precalculated squares in box3/5_h

Instead of calculating squares of pixels once, and shifting and
adding the precalculated squares, just do multiply-accumulate of
the pixels that are shifted anyway for the non-squared sum. This
results in more multiplications in total, but fewer instructions,
and multiplications aren't that much more expensive than regular
arithmetic operations anyway.

On Cortex A53 and A72, this is a fairly substantial gain, on A73
it's a very marginal gain.

The runtimes for the box3/5_h functions themselves are reduced
by around 16-20%, and the overall runtime for SGR is reduced
by around 2-8%.

Before:                   Cortex A53       A72       A73
selfguided_3x3_10bpc_neon:  513086.5  385767.7  348774.3
selfguided_5x5_10bpc_neon:  378108.6  291133.5  253251.4
selfguided_mix_10bpc_neon:  876833.1  662801.0  586387.4

After:                    Cortex A53       A72       A73
selfguided_3x3_10bpc_neon:  502734.0  363754.5  343199.8
selfguided_5x5_10bpc_neon:  361696.4  265848.2  249476.8
selfguided_mix_10bpc_neon:  852683.8  615848.6  577615.0

xiph/rav1e,xiph/rav1e