Avoid splitting fields in the common case. Field splitting was 84% of
the overall CPU.
name old time/op new time/op delta
ParsePorts-6 33.3ms ± 2% 6.3ms ± 4% -80.97% (p=0.000 n=9+10)
name old alloc/op new alloc/op delta
ParsePorts-6 520B ±79% 408B ± 0% -21.49% (p=0.046 n=10+8)
name old allocs/op new allocs/op delta
ParsePorts-6 7.00 ± 0% 7.00 ± 0% ~ (all equal)
Updates tailscale/corp#2566
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
5a9d977c78