GBE: Handle empty basicblock in Instruction selection

GBE: Handle empty basicblock in Instruction selection

I meet a corner case which leads to empty bb.

Lable $12
add %3, %2, 1

and what's more %3 is not used anymore later, so we will not select
instruction for this line of code. Then only Label instruction left
in the bb, which leads to wrong endifLabel used. The fix simply
generate endif instruction if needed at first in matchBasicBlock().

Signed-off-by: Ruiling Song <ruiling.song@intel.com>
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>