net/netip: optimize As4 and As16

name    old time/op  new time/op  delta
As16-8  2.88ns ± 3%  2.16ns ± 3%  -25.19%  (p=0.000 n=15+15)

Fixes #49379
Updates #20859

Change-Id: If4cf58d19ed0e2ac0f179da5c132ed37061e4cb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/361674


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>