RFR: 8367627: C2: Missed Ideal() optimization opportunity with MemBar

[Benoît Maillard]

This PR addresses a missed optimization in `PhaseIterGVN` for `MemBarAcquire` nodes caused by a missing notification during parsing.

The missed optimization in question is the removal of the the `in(MemBarNode::Precedent)` edge for
`MemBarAcquire` nodes when the the `MemBar` is the only user of its input. This was intially
introduced to get rid of unused `Load` nodes that were only kept alive by such an edge.



https://github.com/openjdk/jdk/blob/eeb7c3f2e8e645938d9db0cf61c1d98d751f2845/src/hotspot/share/opto/memnode.cpp#L4254-L4259

In our case, it happens that the `Load` node gets folded to a constant during the initial
`_gvn.transform` call in `GraphKit::make_load`. Because the value is converted before being
returned, we end up with two constant nodes: one `ConL` and one `ConI`. The `ConL` only
has one usage, and this triggers the optimization during verification.


static int test0() {
    var c = new MyClass();
    // the conversion ensures that the ConL node only has one use
    // in the end, which triggers the optimization
    return (int) c.l;
}


The optimization is not triggered earlier during when we apply `_gvn.transform` on the membar,
because it requires `can_reshape`, which is set to `false` in when we call `apply_ideal` in
`PhaseGVN::transform`.

For this reason, we should call `record_for_igvn(membar)` after the `MemBar` is created
and transformed in `GraphKit::insert_mem_bar` to make sure it gets an `Ideal` pass with
`can_reshape` later.


This issue was initially filed for Valhalla, because a condition in `GraphKit::make_load`
prevents its from occurring when boxing elimination is enabled. Boxing elimination is
disabled temporarily in Valhalla (see [JDK-8328675](https://bugs.openjdk.org/browse/JDK-8328675)),
which caused the issue to appear, but by using `-XX:-EliminateAutoBox` it became clear
that the issue was on mainline.

### Testing
- [x] [GitHub Actions](TODO)
- [x] tier1-3, plus some internal testing

Thank you for reviewing!

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

Commit messages:
 - Record in GraphKit::insert_mem_bar_volatile for consistency
 - Improve test and fix
 - Add test

Changes: https://git.openjdk.org/jdk/pull/28448/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28448&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367627
  Stats: 80 lines in 2 files changed: 80 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28448.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28448/head:pull/28448

PR: https://git.openjdk.org/jdk/pull/28448