automatically label beta/stable regression

[matthiaskrgr]

Just an idea:

Glacier could check ICEs with a nightly, beta and stable and old-stable compiler.
If a tracked issue also crashes the latest stable compiler but not old-stable, automatically add the regression-from-stable-to-stable label, etc.

[JohnTitor]

As discussed on https://rust-lang.zulipchat.com/#narrow/stream/227806-t-compiler.2Fwg-prioritization/topic/adding.20labels.20to.20stable-to-stable.20regressions and I commented on #767 (review), I'm going to close this issue.