GBE: We need use exiting block here.
GBE: We need use exiting block here.

According to the API explanation, we should use exiting block instead of
latch block. llvm 3.6 place an assert on this.

v2:
  Use latch block if it is the exiting block, else use exiting block.

Signed-off-by: Ruiling Song <286a174c1339c6eff91abc41c3a1012c95030c95@intel.com>
Reviewed-by: Zhigang Gong <e04a7b9b70b1e4c6318cf117dcd1a9056e14b97a@linux.intel.com>

wdv4758h/beignet,wdv4758h/beignet,zhenyw/beignet,wdv4758h/beignet,zhenyw/beignet,freedesktop-unofficial-mirror/beignet,freedesktop-unofficial-mirror/beignet,zhenyw/beignet,zhenyw/beignet,wdv4758h/beignet,wdv4758h/beignet,zhenyw/beignet,freedesktop-unofficial-mirror/beignet,freedesktop-unofficial-mirror/beignet