inference: allows conditional object to propagate constraint multiple times (#39936)
Currently we always `widenconditional` conditional var state, which
makes us unable to propagate constraints from conditional object
multiple times:
```julia
@test Base.return_types((Union{Nothing,Int},)) do a
b = a === nothing
c = b ? 0 : a # c::Int
d = !b ? a : 0 # d::Int ideally, but Union{Int,Nothing}
c, d
end == Any[Tuple{Int,Int}] # fail
```
This PR keeps conditional var state when the update is came from a
conditional branching, and allows a conditional object to propagate
constraint multiple times as far as the subject of condition doesn't
change. AFAIU this is safe because the update from conditional
branching doesn't change the condition itself.
julia
53f328df - inference: allows conditional object to propagate constraint multiple times (#39936)
