internal/core/adt: add some smaller reducers for issue 2052

This issue was fixed before, but is broken for V3
It is related to some recent changes.

Issue #2052

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I8ff6248a253cb100be4763b45523e24cf434ef74
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1206326
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>