lowering: don't reverse handler order in (pop-handler-list ...)

[topolarity]

We were accidentally emitting a different pop order for Expr(:leave, ...) if you uncomment the nothing below:

let src = Meta.@lower let
    try
        try
            return 1
        catch
        end
    finally
        # nothing # <- uncomment me
    end
end
    println.(filter(stmt->Base.isexpr(stmt, :leave), src.args[1].code))
    nothing
end

[Keno]

Sensible, but the order of arguments to :leave is not supposed to matter.

[topolarity]

I'm pretty sure we have code already that assumes an ordering:

julia/src/codegen.cpp

Line 6318 in 7a76e32

std::tie(scope_to_restore, scope_ptr) = ctx.scope_restore[enter_idx];

Seems pretty reasonable to me, since we're requiring a stack discipline anyway.

[Keno]

Alright fair enough. Let's just document the requirement (in https://github.com/JuliaLang/julia/blob/master/doc/src/devdocs/ast.md) then and ideally we'd check in the verifier also.