Fix waits of REINDEX CONCURRENTLY for indexes with predicates or expressions

As introduced by f9900df5f9, a REINDEX CONCURRENTLY job done for an
index with predicates or expressions would set PROC_IN_SAFE_IC in its
MyProc->statusFlags, causing it to be ignored by other concurrent
operations.

Such concurrent index rebuilds should never be ignored, as a predicate
or an expression could call a user-defined function that accesses a
different table than the table where the index is rebuilt.

A test that uses injection points is added, backpatched down to 17.
Michail has proposed a different test, but I have added something
simpler with more coverage.

Oversight in f9900df5f9.

Author: Michail Nikolaev
Discussion: https://postgr.es/m/CANtu0oj9A3kZVduFTG0vrmGnKB+DCHgEpzOp0qAyOgmks84j0w@mail.gmail.com
Backpatch-through: 14

src/backend/commands/indexcmds.c

@@ -3739,8 +3739,8 @@ ReindexRelationConcurrently(Oid relationOid, ReindexParams *params)
 		save_nestlevel = NewGUCNestLevel();
 
 		/* determine safety of this index for set_indexsafe_procflags */
-		idx->safe = (indexRel->rd_indexprs == NIL &&
-					 indexRel->rd_indpred == NIL);
+		idx->safe = (RelationGetIndexExpressions(indexRel) == NIL &&
+					 RelationGetIndexPredicate(indexRel) == NIL);
 		idx->tableId = RelationGetRelid(heapRel);
 		idx->amId = indexRel->rd_rel->relam;