GBE: optimize post reg allocation instruction scheduling.

GBE: optimize post reg allocation instruction scheduling.

To make the post scheduling working better, I relax the frequency of
the calling of expireGRF when doing register allocation. Thus we can
reduce the physical register conflict and doing the post scheduling.

Another optimization is to insert a pre retire for the instruction
to release those WRITE_AFTER_READ dependency. Write after read will
not bring any hazard, so we can release those register as soon as
the instruction scheduled.

The pre register allocation scheduling is quite different than post
schedlulig, for now, just disable it.

The whole patch could get about 10% perfromance gain with luxmark.

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