Fix a cornor case which dump CaseTestExpr for IS NOT DISTINCT FROM. (#10348)

For below example:
```
CREATE TABLE mytable2 (
    key character varying(20) NOT NULL,
    key_value character varying(50)
) DISTRIBUTED BY (key);

CREATE VIEW aaa AS
SELECT
    CASE mytable2.key_value
        WHEN IS NOT DISTINCT FROM 'NULL'::text THEN 'now'::text::date
        ELSE to_date(mytable2.key_value::text, 'YYYYMM'::text)
        END AS t
    FROM mytable2;

```

mytable2.key_value will cast to type date. For clause `(ARG1) IS NOT
DISTINCT FROM (ARG2)`, this leads ARG1 to become a RelabelType node and
contains CaseTestExpr node in RelabelType->arg.

So when dumping the view, it'll dump extra `CASE_TEST_EXPR` as below
```
select pg_get_viewdef('notdisview3',false);
                               pg_get_viewdef
-----------------------------------------------------------------------------
  SELECT                                                                    +
         CASE mytable2.key_value                                            +
             WHEN (CASE_TEST_EXPR) IS NOT DISTINCT FROM 'NULL'::text THEN ('now'::text)::date+
             ELSE to_date((mytable2.key_value)::text, 'YYYYMM'::text)       +
         END AS t                                                           +
    FROM mytable2;
(1 row)
```

I dig into commit a453004e, if left-hand argument for `IS NOT DISTINCT
FROM` contains any `CaseTestExpr` node, the left-hand arg should be omitted.
`CaseTestExpr` is a placeholder for CASE expression.
Reviewed-by: NPaul Guo <paulguo@gmail.com>

src/backend/utils/adt/ruleutils.c

@@ -7888,7 +7888,11 @@ get_rule_expr(Node *node, deparse_context *context,
 							Node			*lhs = (Node *) linitial(dexpr->args);
 							Node			*rhs = (Node *) lsecond(dexpr->args);
 
-							if (!IsA(lhs, CaseTestExpr))
+							/*
+							 * If lhs contains CaseTestExpr node as placeholder, we should
+							 * omit the lhs for dump
+							 */
+							if (!IsA(strip_implicit_coercions(lhs), CaseTestExpr))
 							{
 								get_rule_expr(lhs, context, false);
 								appendStringInfoChar(buf, ' ');

src/test/regress/expected/case_gp.out

@@ -21,7 +21,7 @@ DROP VIEW IF EXISTS notdisview;
 NOTICE:  view "notdisview" does not exist, skipping
 CREATE OR REPLACE VIEW notdisview AS
 SELECT
-    CASE
+    CASE 'a'::text = 'test'::text
         WHEN 'test' IS NOT DISTINCT FROM ''::text THEN 'A'::text
         ELSE 'B'::text
         END AS t;
@@ -29,7 +29,7 @@ select pg_get_viewdef('notdisview',true);
                                pg_get_viewdef                               
 ----------------------------------------------------------------------------
   SELECT                                                                   +
-         CASE                                                              +
+         CASE 'a'::text = 'test'::text                                     +
              WHEN 'test'::text IS NOT DISTINCT FROM ''::text THEN 'A'::text+
              ELSE 'B'::text                                                +
          END AS t;
@@ -55,6 +55,30 @@ select pg_get_viewdef('notdisview2',true);
     FROM mytable;
 (1 row)
 
+CREATE TABLE mytable2 (
+    key character varying(20) NOT NULL,
+    key_value character varying(50)
+) DISTRIBUTED BY (key);
+DROP VIEW IF EXISTS notdisview3;
+NOTICE:  view "notdisview3" does not exist, skipping
+CREATE OR REPLACE VIEW notdisview3 AS
+SELECT
+    CASE mytable2.key_value
+        WHEN IS NOT DISTINCT FROM 'NULL'::text THEN 'now'::text::date
+        ELSE to_date(mytable2.key_value::text, 'YYYYMM'::text)
+        END AS t
+    FROM mytable2;
+select pg_get_viewdef('notdisview3',false);
+                               pg_get_viewdef                                
+-----------------------------------------------------------------------------
+  SELECT                                                                    +
+         CASE mytable2.key_value                                            +
+             WHEN IS NOT DISTINCT FROM 'NULL'::text THEN ('now'::text)::date+
+             ELSE to_date((mytable2.key_value)::text, 'YYYYMM'::text)       +
+         END AS t                                                           +
+    FROM mytable2;
+(1 row)
+
 CREATE OR REPLACE FUNCTION negate(int) RETURNS int 
 AS 'SELECT $1 * (-1)'
 LANGUAGE sql CONTAINS SQL

src/test/regress/sql/case_gp.sql

@@ -20,7 +20,7 @@ INSERT INTO mytable values (1,2,'t'),
 DROP VIEW IF EXISTS notdisview;
 CREATE OR REPLACE VIEW notdisview AS
 SELECT
-    CASE
+    CASE 'a'::text = 'test'::text
         WHEN 'test' IS NOT DISTINCT FROM ''::text THEN 'A'::text
         ELSE 'B'::text
         END AS t;
@@ -36,6 +36,21 @@ SELECT
     FROM mytable;
 select pg_get_viewdef('notdisview2',true);
 
+CREATE TABLE mytable2 (
+    key character varying(20) NOT NULL,
+    key_value character varying(50)
+) DISTRIBUTED BY (key);
+
+DROP VIEW IF EXISTS notdisview3;
+CREATE OR REPLACE VIEW notdisview3 AS
+SELECT
+    CASE mytable2.key_value
+        WHEN IS NOT DISTINCT FROM 'NULL'::text THEN 'now'::text::date
+        ELSE to_date(mytable2.key_value::text, 'YYYYMM'::text)
+        END AS t
+    FROM mytable2;
+select pg_get_viewdef('notdisview3',false);
+
 CREATE OR REPLACE FUNCTION negate(int) RETURNS int 
 AS 'SELECT $1 * (-1)'
 LANGUAGE sql CONTAINS SQL