RFR [9] (XS): 8179882: C2: Stale control info after cast node elimination during loop optimization pass

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8179882/webrev.00
https://bugs.openjdk.java.net/browse/JDK-8179882

There's cast elimination logic in 
PhaseIdealLoop::split_if_with_blocks_pre() which was introduced by 
JDK-8139771.

The problem with it is that ConstraintCastNode::dominating_cast() relies 
on immediate control info (in(0)) which can get out of sync with what is 
cached in PhaseIdealLoop (get_ctrl()).

The fix is to catch the case when dom_cast doesn't dominate n based on 
info from PhaseIdealLoop and update control info accordingly.

Testing: manual (replayed problematic compilation & eyeballed the IR), 
JPRT, RBT (hs-tier0-comp, in progress).

Best regards,
Vladimir Ivanov

PS: thanks to Roland for helping with the fix.