8238691: C2: turn subtype check into macro node

[Tobias Hartmann]

Hi Roland,

>> http://cr.openjdk.java.net/~roland/8238691/webrev.01/

Very nice.

Here are some minor (style) comments:
- I don't like that find_bottom_mem code but I guess there's no way to avoid it. Please add at least
a comment because without your explanation in the RFR it's not obvious why it's needed.
- Please use the pointer asterisk consistently in new code ("Type* var" vs. "Type *var") and also
remove extensive whitespacing (for example, "sub( const Type *, const Type * )" in subtypenode.hpp)
- macro.cpp:2571 Wrong indentation
- macro.cpp:2740 Just for consistency, the assert message should be the same as above
- subtypenode.hpp:45 should be removed
- In SubTypeCheckNode::Ideal, why are you adding these temporary nodes to the worklist instead of
just calling remove_dead_node? Also, shouldn't you do these before the return in line 143 as well?
- Copyrights should be updated to 2020

> I verified that it fixes JDK-8220708 [1]. 

Can we close JDK-8220708 as duplicate then?

Have you filed an issue for the node budget problem yet?

Thanks,
Tobias