Partial revert "Compiler: refactor and slightly optimize merging two types (#12436)"

[caspiano]

This PR reverts commit bfc33db.

I haven't been able to extract a minimal reproduction of the error discussed on the forum, but I'll work on one.

I'll also review the individual commits made in #12436 with the failing code to see if we can keep any of the optimisations.

[caspiano]

This commit in the original PR introduced the regression

[asterite]

Looks good!

Ideally we'd have a small test that shows what breaks with this change, but I understand it can be hard to do that sometimes.

[caspiano]

Honestly, struggling to come up with a minimal reproduction.
This is the line that causes the issue, crashing on request_value called on the in prepare_call_args_non_external.

This is the method called in the block, #debug(module_id : String, &callback : (String) -> Nil) : Nil. Which looks like it is casting the return type of Channel#send(value : T) : self to Nil

I'll take a look at the different return values of type_merge against the problematic code before and after this PR for further insight and hopefully a test case.

[beta-ziliani]

Thanks @caspiano ! That was fast!