It is likely from when the FTL used LLVM, before B3/Air were written, making this option completely obsolete (and it is currently unused).
Created attachment 388642 [details] Patch
The commit-queue encountered the following flaky tests while processing attachment 388642 [details]: editing/spelling/spellcheck-async-remove-frame.html bug 158401 (authors: morrita@google.com, rniwa@webkit.org, and tony@chromium.org) The commit-queue is continuing to process your patch.
Comment on attachment 388642 [details] Patch Clearing flags on attachment: 388642 Committed r255051: <https://trac.webkit.org/changeset/255051>
All reviewed patches have been landed. Closing bug.
<rdar://problem/58858445>