inference: improve inference of indexing into union of tuples

[aviatesk]

The precision of return type of tuple indexing really relies on constant
propagation, but when an argument is union of tuples, call signature
will be split and constant prop' won't happen, and so inference ends up
looser return type: e.g.

julia> Base.return_types((Union{Tuple{Int,Nothing},Tuple{Int,Missing}},)) do t
           a, b = t
           a # I expected a::Int, but a::Union{Missing,Nothing,Int}
       end |> first
Union{Missing, Nothing, Int64}

This PR special cases some of tuple indexing methods
(getindex(::Tuple) and indexed_iterate(::Tuple)) and forces constant
propagation to happen even when their argument is union of tuples,
by converting it to tuple of unions.

---

The diff certainly looks a bit odd; is there a better way to address this ?

[bramtayl]

Is there a way to fix this more generally? For example, in cases like below:

julia> struct A{B, C}
            b::B
            c::C
        end

julia> Base.return_types(x -> x.b, Tuple{Union{A{Int, Nothing}, A{Int, Missing}}})
1-element Array{Any,1}:
 Union{Missing, Nothing, Int64}

[aviatesk]

Is there a way to fix this more generally?

We need to enable constant prop' for union split signatures to fix this in general.

[bramtayl]

We need to enable constant prop' for union split signatures

Right, is that possible?

[aviatesk]

yeah, later I'll make another PR doing that as an alternative for this PR (but its latency impact should be examined carefully)

[aviatesk]

yeah, later I'll make another PR doing that as an alternative for this PR (but its latency impact should be examined carefully)

Done: #39305

[bramtayl]

Yay!

[aviatesk]

Will be superceded by #39305