RFR: 8219713: Reduce work in DefaultMethods::generate_default_methods

[Claes Redestad]

Hi,

on modern, larger applications, DefaultMethods::generate_default_methods
can show up prominently in startup profiles, so I took a stab at dealing
with some apparent inefficiencies:

- java.lang.Object is scanned at least once for every interface in the
hierarchy - this will be a no-op after the first so we should filter
out Object except the first time we encounter it.

- the resolveNatives and <clinit> are added and checked for every
class/interface scanned, but then explicitly filtered out later on. It
seems that all static initializers and private static methods can be
safely filtered out from the first pass since they wouldn't mask default
methods anyhow.

- a few minor cleanups and simplifications, e.g., removing never
exercised code to cancel and reset an iteration.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8219713
Webrev: http://cr.openjdk.java.net/~redestad/8219713/open.01/

Testing: tier1-3, measured a speed-up on certain startup application

Thanks!

/Claes