Fix assignment to array of domain over composite, redux.

Commit 3e310d837 taught isAssignmentIndirectionExpr() to look through
CoerceToDomain nodes.  That's not sufficient, because since commit
04fe805a1 it's been possible for the planner to simplify
CoerceToDomain to RelabelType when the domain has no constraints
to enforce.  So we need to look through RelabelType too.

Per bug #17897 from Alexander Lakhin.  Although 3e310d837 was
back-patched to v11, it seems sufficient to apply this change
to v12 and later, since 04fe805a1 came in in v12.

Dmitry Dolgov

Discussion: https://postgr.es/m/17897-4216c546c3874044@postgresql.org

src/test/regress/expected/domain.out

@@ -556,6 +556,15 @@ table dcomptable;
  {"(1,5)"}
 (1 row)
 
+-- if there's no constraints, a different code path is taken:
+alter domain dcomptype drop constraint dcomptype_check;
+update dcomptable set f1[1].cf1 = -1;  -- now ok
+table dcomptable;
+     f1     
+------------
+ {"(-1,5)"}
+(1 row)
+
 drop table dcomptable;
 drop type comptype cascade;
 NOTICE:  drop cascades to type dcomptype

src/test/regress/sql/domain.sql

@@ -288,6 +288,10 @@ table dcomptable;
 update dcomptable set f1[1].cf1 = -1;  -- fail
 update dcomptable set f1[1].cf1 = 1;
 table dcomptable;
+-- if there's no constraints, a different code path is taken:
+alter domain dcomptype drop constraint dcomptype_check;
+update dcomptable set f1[1].cf1 = -1;  -- now ok
+table dcomptable;
 
 drop table dcomptable;
 drop type comptype cascade;