Init df for split pass since some target use REG_NOTE in split pattern

[Kito Cheng]

Hi all:

Some target(for example i386, sh and h8300) use find_regno_note in
split pattern but df infrastructure seem not initialize at split pass,
so it may got wrong note since it's out-of-date.

ChangeLog
2016-07-25  Kito Cheng <kito.ch...@gmail.com>

        * gcc/recog.c (split_all_insns): Initialize df for split pass.
        (split_all_insns_noflow): Likewise.