[LWorld] Value type array support for C2 (and lots of bug fixes)

[Roland Westrelin]

> Yes, it's with a constant null return. For example, if getNull (line 430) is compiled:
> http://cr.openjdk.java.net/~thartmann/valhalla/lworld_arrays/webrev.00/test/hotspot/jtreg/compiler/valhalla/valuetypes/TestLWorld.java.html

Then we don't need a null check, right? It's either a ValueTypeNode or
the null constant. Instead of the null check, I think we want an assert
for a null constant and unconditionally an uncommon trap.

Roland.