RFR: 8262046: Clean up parallel class loading code and comments [v6]

[Coleen Phillimore]

> This change reduces the length of SystemDictionary::resolve_instance_class_or_null, the main class loading function to 4 pages on my screen, down from ~6.   Some functions are made static inside systemDictionary.hpp.
> 
> The only executable change is that handle_parallel_super_load used to have a wait while the class's superclass was being loaded after calling resolve_super_or_fail.  It was similar to the subsequent code that waited for the LOAD_INSTANCE placeholder, immediately following the return from handle_parallel_super_load.  I consolidated this code to wait for both conditions and kept the comment from handle_parallel_super_load.  This wait is now in the function handle_parallel_loading.
> 
> I also added a new load_instance_class that calls load_instance_class_impl.  The new load_instance_class does constraint checking and bookkeeping after it is called so that it can return with CHECK_NULL and not check pending exceptions. These functions aren't called outside of systemDictionary.cpp.
> 
> I added a test to show why handle_parallel_super_load is needed (see bug for details of the deadlock that this test would get if not for handle_parallel_super_load).
> 
> This updates comments to:
> 1. rewrote ClassCircularityError detection above resolve_super_or_fail so that it made more sense to me (hope it does for others too).
> 2. move comments to be near the code they describe
> 3. removed some comments that referred to code somewhere else but not helpful at that location, or to describe code that was deleted or obsolete, or repetitive
> 4. added a comment for resolve_instance_class_or_null
> 
> Ran tier1-3 testing, jck vm, jck lang and our internal parallel class loading tests.  Retesting with tiers 4-8 in progress (I tested an earlier version of this with no failures).

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  fix THREAD

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3200/files
  - new: https://git.openjdk.java.net/jdk/pull/3200/files/4f1b42d5..5cf6aa61

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3200&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3200&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3200.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3200/head:pull/3200

PR: https://git.openjdk.java.net/jdk/pull/3200