internal/core/adt: fix structural cycle stack overflow

We now only allow n.hasNonCycle to be set for
non-cyclic conjuncts.

Let expressions are not yet covered by this
mechanism and so we exclude that exemption.

Fixes #3731

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I86eb50a8675a00ac1217ead480d2078c3047abcb
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1209389
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mpvl