[AArch64] Convert concat(uhadd(a,b), uhadd(c,d)) to uhadd(concat(a,c), concat(b,d)) (#79464)

We can convert concat(v4i16 uhadd(a,b), v4i16 uhadd(c,d)) to v8i16
uhadd(concat(a,c), concat(b,d)), which can lead to further
simplifications.