RFR: 8306922: IR verification fails because IR dump is chopped up

[Emanuel Peter]

Some IR tests were failing because the logs were messed up (badly interleaved), due to multiple threads writing at the same time.

The issue was in `Compile::print_ideal_ir`. While we did lock the `tty`, this lock was broken because the node dumping sometimes calls into the VM, which can trigger Safepoints, which in turn can explicitly break the locks, with `ttyLocker::break_tty_lock_for_safepoint`.

What good is a lock if it can be broken? Not much. Though it is only broken if there is a Safepoint, so if we can avoid safepointing while holding the lock we should be safe. To verify that, we can write:


  NoSafepointVerifier nsv;
  ttyLocker ttyl;


The verifier triggered immediately, as expected.
And I now gather the whole dump into a separate `stringStream` first, so that all the Safepoints can happen before I even acquire the lock. Then, I acquire the log and just print the `stringStream` to `tty / xtty`, without triggering any Safepoint.

We still need to acquire a lock, else other threads may be printing also, and we get a bad interleaving in the locks.

I had to enable `dump_bfs` to print to an arbitrary stream, instead of just `tty`.

I un-problemlisted `compiler/c2/irTests/TestVectorConditionalMove.java`, and fixed a CompileCommand issue for it.

**Question**

Maybe we should in general go through all uses of `ttyLocker`, and add a `NoSafepointVerifier` to ensure no such lock is broken? Or maybe we just build the verifier into the ttyLocker? Should I file an RFE?

**Testing**

I ran `TestVectorConditionalMove.java` 1000x on master, just with the CompileCommand issue fixed. It triggered an IR issue 5 times (hit rate `0.005`). With the fix, it never triggers (`0.995^1000 = 0.007`). I also ran up to tier6 and stress-testing.

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

Commit messages:
 - renamed _st to _output
 - make sure CloneMap info is also printed to stream, and not to tty
 - add locker again, and refactor
 - write to xtty or tty
 - add tty locker again
 - buffer xtty and tty
 - 8306922: IR verification fails because IR dump is chopped up

Changes: https://git.openjdk.org/jdk/pull/14591/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14591&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306922
  Stats: 168 lines in 8 files changed: 24 ins; 4 del; 140 mod
  Patch: https://git.openjdk.org/jdk/pull/14591.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14591/head:pull/14591

PR: https://git.openjdk.org/jdk/pull/14591