AArch64: Optimize Armv8.0 Neon path of HBD horizontal 6-tap filters

The 6-tap horizontal subpel filters can be further improved by some
pointer arithmetic and saving some instructions (EXTs) in their data
rearrangement codes.

Relative runtime of micro benchmarks after this patch on some Cortex
CPU cores:

regular:     X1      A78      A76      A55
 mc  w8:  0.915x   0.937x   0.900x   0.982x
 mc w16:  0.917x   0.947x   0.911x   0.971x
 mc w32:  0.914x   0.938x   0.873x   0.961x
 mc w64:  0.918x   0.932x   0.882x   0.964x