RFR 8223110 [lworld] Handle GC that happens when C1 allocates buffered value objects

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8223110
http://cr.openjdk.java.net/~iklam/valhalla/8223110-gc-while-buffering-objs-in-c1-vep.v01/

Here's a small fix that properly scans oops on the callstack when GC 
happens while the C1 entry points allocate buffered instances of inlined 
classes.

I also fixed a related bug in 
CompiledMethod::preserve_callee_argument_oops where we should use the 
scalarized calling convention only for C2-compiled methods.

With this patch, C1 can basically handle incoming scalarized parameters. 
There are still some bugs that I need to fix (see commented out cases in 
TestCallingConventionC1.java). I'll also start working on returning 
scalarized values from C1-compiled methods.

Thanks
- Ioi