[lworld] RFR JDK-8207259: Ensure VarHandle compare and set/exchange work for value types

[mandy chung]

Webrev:
    http://cr.openjdk.java.net/~mchung/valhalla/webrevs/8207259/webrev.01

This patch fixes the compare and set/exchange to work properly for
flattened, and non-flattened value types properly.  The new VarHandle
access tests cover flattened, non-flattened value fields and arrays and
VarHandle whose symbolic type is Object[] writing on a flattened array.

- Unsafe::compareAndSetObject and compareAndExchangeObject may be
   called for writing to a non-flattened value field.  In that case,
   it should perform the substitutability check (rather than identity
   check).  We probably need some unsafe API to detect if the operands
   are identityless.  For now, Unsafe::compareAndSetObject and
   compareAndExchangeObject take an additional field type parameter
   and determine if it's identityless based on the field type and
   the expected object.

- refactor VarHandleObjects to split the support for flat value and
   generate from X-VarHandle.java.template a new class VarHandleValues
   at build time.   I think this is a cleaner way to handle values.
   VarHandleObjects provides VarHandle support for references including
   non-flattened value fields that uses Unsafe reference access e.g
   getObject/putObject whereas VarHandleValues provides VarHandle
   support for flattened value fields that uses Unsafe flat value access.

   (I'm working on a separate patch to rename xxxObject to xxxReference
    to make the Unsafe API explicit).

- VarHandleObjects.Array has to handle Object[] as the symbolic type
   but the array instance is a flattened value type that might have
   the performance impact for VarHandle for reference types.  The
   implementation tests if VarHandle symbolic type is Object[]
   and the array object is flattened array and hence the impact
   hopefully is limited but will wait for performance number to tell.

- The new VarHandleTestAccessPoint test crashes (SIGSEGV) when running
   with compiler.  I will file a JBS issue separately.  So the test
   for now will run only with -Xint.

Thanks
Mandy