Support invoke backedges for const (fix #44320)

[timholy]

This is a left-over piece of #46010, triggered by const args.

I am not certain this is all of them, might there be other paths to add_backedge! that we're not passing down invoketypes to? The issue is that in the case of an invoke backedge, not only do we have to add the proper signature-"decorated" backedge, we have to be sure we don't add the undecorated variant. Otherwise the caller will be invalidated even though the decorated backedge remains valid.

Fixes #44320

[giordano]

Note that writing "fix #issue" won't actually link the PR to the issue. The magic text should go into the commit message body and/or the description of the PR. GitHub doesn't even linkify "fix #issue" in the title of a PR, so doing that will make it harder to actually visit the mentioned issue.