RFR(M): 8188770: [MVT] Various code cleanups

[Tobias Hartmann]

Hi,

please review the following cleanup change:
https://bugs.openjdk.java.net/browse/JDK-8188770
http://cr.openjdk.java.net/~thartmann/8188770/webrev.00/

Here's a list of the major changes:
- I've re-enabled the -XX:-ValueArrayFlatten test configuration I accidentally removed with 8185556
- By default, Type::get_const_type() -> TypeOopPtr::make_from_klass_common() returns a TypeValueTypePtr with type 
TypePtr::NOTNULL. We then cast it to bottom if necessary. I've changed the method to return bottom by default to make it 
consistent with other types and added NOTNULL casts to the callers where necessary.
- I've implemented value type pointer flattening (see changes in compile.cpp). This was necessary because Bottom and 
NotNull types didn't alias to them same slice causing incorrect results.
- Resolved remaining TODOs from 8185556
- Renamed ValueTypeNode and ValueTypePtrNode 'make' methods and cleaned up unused arguments
- Removed TypeValueTypePtr::_vt which was only used to retrieve the value klass
- Other small refactorings (for example, GraphKit::initialize_value_type_array) and code cleanups

All tests pass.

Thanks,
Tobias