RFR: 8315088: C2: assert(wq.size() - before == EMPTY_LOOP_SIZE) failed: expect the EMPTY_LOOP_SIZE nodes of this body if empty

[Roland Westrelin]

The assert fires in code that looks for counted loops that would be
empty if not part of a loop strip mining nest (that is some nodes in
the the loop body are kept alive because of the safepoint in the outer
loop, the loop would otherwise be empty). That logic starts by finding
the set of `EMPTY_LOOP_SIZE` nodes that are core to all counted loops
(exit test, iv phi etc) and store them in the `empty_loop_nodes`
list.

Finding the "core" nodes is performed by
`IdealLoopTree::collect_loop_core_nodes`. It enqueues the backedge of
the loop in `empty_loop_nodes` and then iteratively pushes inputs of
nodes in `empty_loop_nodes` that belongs to the loop until it can't
make progress. There should be `EMPTY_LOOP_SIZE` of those nodes.

 It's possible that that 2 or more loops are involved in the process:
a node n in loop 1 could be kept alive by the safepoint of loop 2 so
loop 2 needs to be empty for n to become dead and possibly loop 1 to
be empty. When that happens while the logic is in the process of
determining if loop 1 is empty, it needs to also collect the
`EMPTY_LOOP_SIZE` nodes that make the "core" of loop 2. They are
enqueued to the `empty_loop_nodes` list too.

The failure happens when `empty_loop_nodes` already has the "core"
nodes of loop 1 and "core" nodes for loop 2 are collected. In the
process`IdealLoopTree::collect_loop_core_nodes` iterates on all
`empty_loop_nodes` (loop 1 nodes included, not just starting from the
loop 2 backedge). That's inefficient (loop 1 nodes can't help find
nodes that belong to loop 2) and wrong because there could be an edge
from a node in loop 1 to a node in the body of loop 2. That node in
loop 2 is them pushed to the `empty_loop_nodes` list. 

The fix I propose is to only iterate over loop 2 nodes when loop 2 is
processed.

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

Commit messages:
 - test
 - fix

Changes: https://git.openjdk.org/jdk/pull/15520/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15520&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315088
  Stats: 54 lines in 2 files changed: 53 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15520.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15520/head:pull/15520

PR: https://git.openjdk.org/jdk/pull/15520