V4 IR: do edge splitting after SSA transformation

V4 IR: do edge splitting after SSA transformation

This reduces the work for the dominator tree/frontier calculations,
because there are less blocks to consider. All blocks inserted by
splitting the critical edges, have (by definition) no effect on the
dominator calculations. However, the immediate dominators for all new
blocks needs to be added, because this information is used by the block
scheduling.

This change reduces memory/time usage during optimization passes,
especially when processing excessively big switch statements.

Change-Id: Ia69882e9dabdddffa1c98b1079012d8d988e1e8f
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>