re_intuit_start(): document floating code better

re_intuit_start(): document floating code better

Add some comments to the float-finding code that explains what the t, last
and last1 vars are, and how they're calculated.

Also make the calculation of last separate from last1;
it should logically be the same, but clearer. i.e. change

    last = last1 = ...;
    if (cond)
        last = ....;

to

    last1 = ...;
    last = cond ? .... : last1;