RFR: 8252406: Introduce Thread::as_Java_thread() convenience function

[David Holmes]

This is a rather large but generally simple cleanup.

We use a lot of raw C-style casts to "(JavaThread*)" typically after checking "thread->is_Java_thread()". To simplify
this pattern, and make the code look somewhat cleaner we introduce a convenience function Thread::as_Java_thread(),
which asserts is_Java_thread() and returns "this" cast to "JavaThread*". A const version, as_const_Java_thread(), was
also added to allow use in cases where we start with "const Thread *".

Summary of changes:
- changed raw casts to use as_Java_thread()
- removed redundant checks of is_Java_thread() as it is now done in as_Java_thread()
- Removed checks that are checking the type system e.g.
void foo(JavaThread* t) {
  assert(t->is_Java_thread(), "must be")
the only way the assert can fire is if someone deliberately bypasses the type system, and if we are going to worry
about that then we would need asserts like this on every method that expects a JavaThread! The right place for the
assertion is at the head of any such call chain.

- Removed asserts that are already guaranteed in the caller.
- Use JavaThread::current() where appropriate.
- Use pre-existing "thread" variable where available rather than casting THREAD

Change locations found by grepping for variations of the cast syntax "(JavaThread*)" - it is possible some may have
been missed.

Testing: tiers 1-3

Thanks,
David

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Commit messages:
 - Initial changes for 8252406

Changes: https://git.openjdk.java.net/jdk/pull/37/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=37&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252406
  Stats: 463 lines in 112 files changed: 15 ins; 124 del; 324 mod
  Patch: https://git.openjdk.java.net/jdk/pull/37.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/37/head:pull/37

PR: https://git.openjdk.java.net/jdk/pull/37