RFR(M): 8027422: assert(_gvn.type(obj)->higher_equal(tjp)) failed: cast_up is no longer needed

[Roland Westrelin]

The root of the problem is that during the null check when the type of obj is improved in GraphKit::cast_not_null():
const Type *t_not_null = t->join(TypePtr::NOTNULL, true);
The join with TypePtr::NOTNULL is not applied to the speculative part. In fact, no meet between a TypeOopPtr and a TypePtr modifies the speculative part. One way to fix it would be to apply the meet with a TypePtr to the speculative part as well as the standard part of the type which I tried: then we need to move the _speculative field up in TypePtr and modify all operations on TypePtr to operate on _speculative so that the type system remains symmetric.
In many places where we mix a TypePtr with a TypeOopPtr we actually don’t care about the speculative part. I changed the following operations on Type:
higher_equal()
meet()
join()
filter()
so that by default they don’t return a result that include the speculative part of the type. Where we need the speculative part of the type, we have to explicitly request it.

I also fixed a problem with Type nodes with a _type of TypeNarrowOop that wouldn’t drop the speculative part of the type during Compile::remove_speculative_types().
I included small clean ups that Mikael suggested privately (dropped the duplicate check for res->isa_oopptr() in TypeOopPtr::meet, make remove_speculative not go through the exercise of creating a new type if speculative is NULL).

http://cr.openjdk.java.net/~roland/8027422/webrev.00/

Roland.