inference: forward Conditional inter-procedurally

[aviatesk]

The PR #38905 only "back-propagates" conditional constraint
(from callee to caller), but currently we don't "forward" it
(caller to callee), and so inter-procedural constraint propagation
won't happen for e.g.:

ifelselike(cnd, x, y) = cnd ? x : y
@test Base.return_types((Any,Int,)) do x, y
    ifelselike(isa(x, Int), x, y)
end |> only == Int

This commit complements #38905 and enables further inter-procedural
conditional constraint propagation by forwarding Conditional to
callees when it imposes a constraint on any other argument,
during constant propagation.

---

• IIUC, this PR will be a requirement of Make Base.ifelse a generic function #37343 ?
• /cc @c42f @chriselrod

[c42f]

This seems great! I had a read through though I don't know the surrounding code well enough to review properly.

[aviatesk]

Alright, I think this PR is ready to go. @vtjnash can I ask your review on this ?

[aviatesk]

@vtjnash I'd like to merge this PR tomorrow on my responsibility, but I'd appreciate if you could review it if you have a time.

[vtjnash]

I don't believe this is true, since Conditional is actually tri-stated. Consider:

f(::Int) = 1
f(::Float64) = 2
f(::String) = 3

x::Any
if x isa Union{Int, Float64}
   cond = x isa Int
end
f(cond, x) # x was Any before examining `cond::Conditional(:x, Int, Float64)`, but now the signature for `String` is excluded

[aviatesk]

Right, thanks for your review.

I added 28f50c6 so that we just propagate Bottom in such a case (where we accidentally prove the method match is impossible).
Now this code snippet doesn't throw:

Base.@constprop :aggressive f(cnd, ::Int) = cnd
Base.@constprop :aggressive f(cnd, ::Float64) = !cnd
Base.@constprop :aggressive f(cnd, ::String) = cnd

function g()
    x = v::Any
    if x isa Union{Int, Float64}
        cnd = x isa Int # ::Conditional(:x, Int, Float64)
    end
    f(cnd, x)
end

@code_typed optimize=false g()