Generator Type Instability

```julia
function t1()
    p=zeros(2)
    m=minimum(p[i] for i=1:2)
    p=zeros(2)
    return m
end
function t2()
    p=zeros(2)
    m=minimum(p[i] for i=1:2)
    return m
end
```

`@code_warntype t1()` will show it is type unstable. However, `t2` is type stable.

Maybe related to #18695. But looks strange because `t1` and `t2` are indeed the same.