GBE: fix the wrong usage of stack pointer and stack buffer.

GBE: fix the wrong usage of stack pointer and stack buffer.

Stack pointer and stack buffer should be two different virtual
register. One is a vector and the other is a scalar. The reason
previous implementation could work is that it search curbe offset
and make a new stack buffer register manually which is not good.
Now fix it and remove those hacking code. We actually don't need
to use curbe offset manually after the allocation.

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