Without this change, the compiler fails to notice that `env_threads isa
Int` in the fall-through case, leading to a union-split with a branch
that is in fact unreachable:

```
43 ┄ %109 = φ (mmtk#41 => %105, mmtk#42 => %108)::Union{Nothing, Int64}
│    %110 = (%109 isa Int64)::Bool
└───        goto mmtk#45 if not %110
...
45 ─ %126 = π (%109, Nothing)
│           Base.convert(Int64, %126)::Union{}
└───        unreachable
```

After this change, the union-split is eliminated.

Co-authored-by: Jeff Bezanson <[email protected]>

…g#59889)

Stdlib: ArgTools
URL: https://github.com/JuliaIO/ArgTools.jl.git
Stdlib branch: master
Julia branch: master
Old commit: 1314758
New commit: 89d1959
Julia version: 1.13.0-DEV
ArgTools version: 1.1.2(Does not match)
Bump invoked by: @IanButterworth
Powered by:
[BumpStdlibs.jl](https://github.com/JuliaLang/BumpStdlibs.jl)

Diff:
JuliaIO/ArgTools.jl@1314758...89d1959

```
$ git log --oneline 1314758..89d1959
89d1959 Merge pull request mmtk#41 from JuliaIO/dependabot/github_actions/actions/checkout-5
c0bdfea build(deps): bump actions/checkout from 4 to 5
```

Co-authored-by: IanButterworth <[email protected]>