Fix typo and avoid possible memory leak in average_num_loop_insns

Fix typo and avoid possible memory leak in average_num_loop_insns

Function average_num_loop_insns forgets to free loop body in early
return.  Besides, overflow comparison checks 1000000 (e6) but the
return value is 100000 (e5), fix this typo.

gcc/ChangeLog

2020-01-14  Kewen Lin  <linkw@gcc.gnu.org>

    * cfgloopanal.c (average_num_loop_insns): Free bbs when early
    return, fix typo on return value.