Improve VarHandle checks for interpreter

- Flatten TypesAndInvokers: this adds 4-8 bytes to VarHandle in some configurations, but reduces indirection and improve laziness and net total footprint (in-use VHs will typically always need the MethodType[], but might never need the MethodHandle[])
- Have checkExactAccessMode return the "direct" predicate of the VarHandle, so that VarHandleGuards can avoid some method calls

Both speed-up invocation of VHs in interpreted mode.