internal/core/adt: fix cycle in patterns

The finalized state of a node was set before processing
patterns. This caused cycles in patterns to go unnoticed.

Fixing this further exposed some issues with cycle detection
in sharing: allowing cyclic nodes to share could make
structure sharing go unnoticed. We now do not allow
sharing in this case.

Another issue was that clearing the Refs in a nonRooted
struct could remove a necessary cycle detection.

Issue #3476
Issue #2854
Issue #2850

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: Ibfbf716b9aa2b07ac83d0125a8e5188486eda4eb
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202216
Reviewed-by: Matthew Sackman <matthew@cue.works>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cue/testdata/benchmarks/issue1684.txtar

@@ -62,15 +62,15 @@ nestedClosed: passing: {
 	D: {id: {}} | {[string]: D}
 }
 -- out/evalalpha/stats --
-Leaks:  2135
-Freed:  530
-Reused: 530
-Allocs: 2135
+Leaks:  3341
+Freed:  794
+Reused: 794
+Allocs: 3341
 Retain: 0
 
-Unifications: 475
-Conjuncts:    4333
-Disjuncts:    790
+Unifications: 937
+Conjuncts:    6553
+Disjuncts:    1414
 -- out/evalalpha --
 (struct){
   #Secret: (#struct){
@@ -143,18 +143,18 @@ diff old new
 -Freed:  1064333
 -Reused: 1064282
 -Allocs: 51
-+Leaks:  2135
-+Freed:  530
-+Reused: 530
-+Allocs: 2135
++Leaks:  3341
++Freed:  794
++Reused: 794
++Allocs: 3341
  Retain: 0
 
 -Unifications: 792123
 -Conjuncts:    2480117
 -Disjuncts:    1064333
-+Unifications: 475
-+Conjuncts:    4333
-+Disjuncts:    790
++Unifications: 937
++Conjuncts:    6553
++Disjuncts:    1414
 -- diff/-out/evalalpha<==>+out/eval --
 diff old new
 --- old