GBE: handle dead loop BBs in liveness analysis.

GBE: handle dead loop BBs in liveness analysis.

Considering the following CFG, our previous liveness analysis will only
back traverse from the exit point BB, thus the BB 6 and 7 will not be handled.

    4---
    |   |
    |   |
    5 --|---> 10 ---> ret
    |   |
    |   |
    6<--
    |
    |<--
    7   |
    |   |
     ---

Although the CFG looks not a normal application as once it goes to
block 6, it will enter a dead loop and will never return, we still need
to compile it successfully.

This patch is to fix the bug at:
https://bugs.freedesktop.org/show_bug.cgi?id=85362

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>
Reviewed-by: "Yang, Rong R" <rong.r.yang@intel.com>