extract_muldiv bug

[Richard Kenner]

    The problem seems to be in extract_muldiv, but before fixing it I'd
    like to know whether convert()/extract_muldiv()/fold() etc. functions
    are allowed to destroy its arguments if returning non-NULL or not,
    ie. if one does:

    t1 = convert (sometype, t);
    and t1 is != NULL, can t1 be thrown away and returned original t or not?

I'd say "yes".  convert is not supposed to modify its operand under
any conditions.  Trees are allowed to be arbitrarily shared.