Request for reviews (S): 6827505: sizing logic for vtable and itable stubs needs self-check

[John Rose]

Here are some simple asserts and comments which should make it easier  
to maintain the correct sizing of certain stubs:
   http://cr.openjdk.java.net/~jrose/6827505/webrev.00/

This is a follow-up to my push regarding itable stubs, which changed  
the sizing:
   http://cr.openjdk.java.net/~jrose/6812831/webrev.01/

On retrospect, I wasn't satisfied with the robustness of the sizing  
logic.  Clearly something more automatic could be done, but I'll leave  
that for someone else to do.  Meanwhile, these comments and asserts  
are safe and simple.

-- John