RFR (S): 8071996: split_if accesses NULL region of ConstraintCast

[Lindenmaier, Goetz]

Hi,

I fixed a small bug in the C2 compiler.
It appeared in our s390/zArch port running hs25.31 with jdk6 when executing
jvm2008/compiler.compiler.

Split_if encounters a ConstraintCast without region edge, which is a valid pattern.
But it reads the region edge unconditionally, thus failing.

Please review this change.  I please need a sponsor.
http://cr.openjdk.java.net/~goetz/webrevs/8071996-fixSplitIf/webrev.01/

Best regards,
  Goetz.