Review Request JDK-8206121: [lworld] ensure non-nullable on setting value field via reflection and VarHandle

[mandy chung]

Webrev at:
   http://cr.openjdk.java.net/~mchung/valhalla/webrevs/8206121/webrev.00

Add null check in value field access in LambdaForm for DMH and 
VarHandle.  For DMH, a new FT_CHECK_VALUE kind indicates that
a value field requires a null check.  A value may be flattened
or non-flattened that will use the linker method with and without
the field type.  So adjust the way to compute the index to the
ACCESSOR_FORMS cache as it will have a different LF for flattened
and non-flattened value field.  The test verifies nullability
and immutability.

Mandy