test: Reduce amount of code generated for test20855 again

[ibuclaw]

See introducing PR:

• fix Issue 20855 - stack overflow when compiling large file #12409

See pull request that subsequently reduced the amount of code generated due to CI failures:

• test20855 is too much for some platforms #12454

See possibly related bug report:

• https://issues.dlang.org/show_bug.cgi?id=22098

See pipelines that have been failing since this morning in master:

• https://cirrus-ci.com/task/4605506372239360
• https://cirrus-ci.com/task/5975755894030336
• https://cirrus-ci.com/task/5629715546374144

I've tried reverting dmd to 8884ad0, druntime to 934b4d2, and phobos to 21227390e (all merge_stable PRs), and the test failure still occurs. Conclusion is that there's an environmental change in the FreeBSD 11 images that affects this test, and this test only. But why that should be a problem for dmd means that either the dmd fix is wrong, or dmd is incapable of generating correct code for the given test.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

Bugzilla references

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#12985"