RFR(XS): ZGC: C2: Missing null check in LoadBarrier::bottom_type

[Nils Eliasson]

Hi,

Please review,

Stefan reported a crash when using -XX:+TraceIterativeGVN. When printing 
the types of the nodes, a loadbarrier is encountered that no longer has 
a preceding load (in(oop) is null), and we crash when we try to call 
bottom_type on that.

We don't see this in normal runs because all useless barriers are 
eliminated before checking types. That is not the case when debug printing.

Fix returns top (no possible value) if in_oop is null.

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

Regards,

Nils