syntax: simplify byte set checks
syntax: simplify byte set checks

It even makes some cases a little faster.

name                   old time/op    new time/op    delta
Parse/Vars+Clauses-4     5.32µs ± 1%    5.21µs ± 1%  -2.05%  (p=0.002 n=6+6)

mvdan/sh,mvdan/sh,mvdan/sh,mvdan/sh