RFR (S) JDK-8223017: [lworld] checkcast/instance broken for LW2 arrays

[David Simms]

Here's a fix for the missing super type relationship for null-free 
arrays, i.e. "[QFoo;" <: "[LFoo;"...once LW2 array support went in, it 
accidentally skipped it's null-allow type and went straight to 
Object[]...added a few more test cases.

Webrev:

http://cr.openjdk.java.net/~dsimms/valhalla/8223017/

Bug:

https://bugs.openjdk.java.net/browse/JDK-8223017