GBE: Optmize phi elimination

GBE: Optmize phi elimination

During phi elimination, we simply insert 3 MOVs for one phi instruction
to avoid lost copy issue. But in fact, only two of them are needed for
most of time. This patch tries to see whether the move from phiCopy
to phi can be avoided.

The patch basically checks whether the phiCopy and phi have live range
interference. If no, then they can be coalesced, thus one instruction
can be optimized.

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