[Backport, AVR, 4.6]: PR50289

[Georg-Johann Lay]

Commited the following backport of PR50289 from mainline to 4.6:

http://gcc.gnu.org/viewcvs?view=revision&revision=178529

        PR target/50289
        Backport from mainline r178528
        * config/avr/avr.c (sequent_regs_live): Don't recognize sequences
        that contain global register variable.