wgengine/filter: twiddle bits to optimize

Part of #19.

name            old time/op    new time/op    delta
Filter/icmp4-8    32.2ns ± 3%    32.5ns ± 2%     ~     (p=0.524 n=10+8)
Filter/icmp6-8    49.7ns ± 6%    43.1ns ± 4%  -13.12%  (p=0.000 n=9+10)

Signed-off-by: David Anderson <danderson@tailscale.com>
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