fix regression in debug codegen

[dsyme]

PR #11957 caused a regression in debug code generation in an obscure case used by the debug-mode compiler

For a loop

let unsolved = [true]
let f() = 

        for priority = 0 to 10 do
            unsolved |> List.iter (fun tp -> System.Console.WriteLine())

The pipeline was being incorrectly marked as "has no effect". Normally effect information doesn't matter in debug code generation, however in one case (for loops) the construct was being omitted altogether if the body was judged to have no effect.

This meant the debug-mode compiler we build in our tree was malfunctioning. This fixes this problem.

@brettfo We should probably run at least one of our test suites in debug mode.

[dsyme]

This is ready @KevinRansom @TIHan @vzarytovskii