p2p/enode: optimize DistCmp

[cuiweixie]

goos: darwin
goarch: arm64
pkg: github.com/ethereum/go-ethereum/p2p/enode
cpu: Apple M1 Pro
           │   old.txt    │               new.txt               │
           │    sec/op    │   sec/op     vs base                │
DistCmp-10   12.225n ± 1%   2.772n ± 3%  -77.32% (p=0.000 n=10)

[fjl]

Hey @cuiweixie, I've changed this PR and your other one to remove extra variables. They are not necessary, since the length of type ID is constant (it is an array type), and thus all arithmetic like len(a)+8 will be performed at compile time.