parse: remove some redundancy in advance()
parse: remove some redundancy in advance()

ok      github.com/mvdan/sh     9.507s
name                 old time/op    new time/op    delta
Parse/Whitespace-4     2.63µs ± 1%    2.61µs ± 2%    ~             (p=0.240 n=6+6)
Parse/Comment-4        1.62µs ± 3%    1.64µs ± 1%    ~             (p=0.398 n=6+5)
Parse/LongLit-4        4.66µs ± 1%    4.66µs ± 2%    ~             (p=0.879 n=5+6)
Parse/Cmds-4           19.8µs ± 3%    19.6µs ± 1%    ~             (p=0.310 n=6+6)
Parse/Quoted-4         3.90µs ± 3%    3.83µs ± 0%    ~             (p=0.093 n=6+6)
Parse/NestedStmts-4    11.7µs ± 1%    11.5µs ± 1%  -1.43%          (p=0.002 n=6+6)
Parse/Clauses-4        8.31µs ± 1%    8.25µs ± 2%    ~             (p=0.240 n=6+6)
Parse/Binary-4         7.85µs ± 1%    7.82µs ± 2%    ~             (p=0.394 n=6+6)
Parse/Redirect-4       9.66µs ± 1%    9.70µs ± 3%    ~             (p=0.937 n=6+6)
Parse/Arithm-4         7.49µs ± 2%    7.35µs ± 2%  -1.85%          (p=0.041 n=6+6)

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