8338534: GenShen: Handle alloc failure differently when immediate garbage is pending

[kdnilsen]

Several changes are implemented here:

1. Re-order the phases that execute immediately after final-mark so that we do concurrent-cleanup quicker (but still after concurrent weak references)
2. After immediate garbage has been reclaimed by concurrent cleanup, notify waiting allocators
3. If an allocation failure occurs while immediate garbage recycling is pending, stall the allocation but do not cancel the concurrent gc.

---

Progress

• [x] Change must not contain extraneous whitespace
• [x] Commit message must refer to an issue
• [ ] Change must be properly reviewed (1 review required, with at least 1 Committer)

Issue

• JDK-8338534: GenShen: Handle alloc failure differently when immediate garbage is pending (Bug - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/shenandoah.git pull/479/head:pull/479
$ git checkout pull/479

Update a local copy of the PR:
$ git checkout pull/479
$ git pull https://git.openjdk.org/shenandoah.git pull/479/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 479

View PR using the GUI difftool:
$ git pr show -t 479

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/shenandoah/pull/479.diff

Webrev

Link to Webrev Comment