RFR: C2: Add loop strip mining support to split_barrier_thru_phi

[Nils Eliasson]

Hi,

This patch adds loop strip mining support to 
PhaseIdealLoop::split_barrier_thru_phi.

In short - PhaseIdealLoop::split_barrier_thru_phi is an optimization 
that moves a loadbarrier up the control flow, past merges, by cloning it 
on all incoming edges. This applies to both normal control flow and 
loops. The loop strip mining optimization adds an outer loop on all 
counted loops. When moving barriers out to the loop entry path, on a 
strip mined counted loop, we need to make sure to move it up past the 
outer loop too.

webrev: 
http://cr.openjdk.java.net/~neliasso/zgc/split_barrier_thru_phi/webrev.01/

Regards,

Nils