gcse.c (find_avail_set): Follow chains of register-register copies.

        * gcse.c (find_avail_set): Follow chains of register-register copies.
        Use oprs_not_set_p to guarantee that the returned value can be
        substituted.
        (cprop_insn): Don't verify the return value of find_avail_set with
        oprs_not_set_p.

From-SVN: r28835