cut: reduce CPU overhead in determining item to output

print_kth() is the central function of cut used to
determine if an item is to be output or not,
so simplify it by moving some logic outside.
Benchmark results for this change are:

$ yes abcdfeg | head -n1MB > big-file
$ for c in orig split; do
    src/cut-$c 2>/dev/null
    echo -ne "\n== $c =="
    time src/cut-$c -b1,3 big-file > /dev/null
  done

== orig ==
real    0m0.111s
user    0m0.108s
sys     0m0.002s

== split ==
real    0m0.088s
user    0m0.081s
sys     0m0.007s

* src/cut.c (print_kth): Refactor a branch to outside the function.
Related to http://bugs.gnu.org/13127

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