ART: Better SSA Allocation when recreating SSA

The SSA calculation is not allocation friendly. This makes the
SSARepresentation remember how much is allocated and not reallocate
if SSA should be recalculated.

Also added some allocation friendly code for the dominance code.

Signed-off-by: Jean Christophe Beyler <jean.christophe.beyler@intel.com>
Change-Id: I6418b402434bd850b45771c75b7631b7b84a8f66