Reformat code in ATPostAlterTypeParse.

The code in ATPostAlterTypeParse was very deeply indented, mostly because
there were two nested switch-case statements, which add a lot of
indentation. Use if-else blocks instead, to make the code less indented
and more readable.

This is in preparation for next patch that makes some actualy changes to
the function. These cosmetic parts have been separated to make it easier
to see the real changes in the other patch.

src/backend/commands/tablecmds.c

@@ -8645,9 +8645,9 @@ ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId, char *cmd,
 		Node	   *stm = (Node *) lfirst(list_item);
 		AlteredTableInfo *tab;
 
-		switch (nodeTag(stm))
-		{
-			case T_IndexStmt:
+		tab = ATGetQueueEntry(wqueue, rel);
+
+		if (IsA(stm, IndexStmt))
 		{
 			IndexStmt  *stmt = (IndexStmt *) stm;
 			AlterTableCmd *newcmd;
@@ -8655,38 +8655,39 @@ ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId, char *cmd,
 			if (!rewrite)
 				TryReuseIndex(oldId, stmt);
 
-					tab = ATGetQueueEntry(wqueue, rel);
 			newcmd = makeNode(AlterTableCmd);
 			newcmd->subtype = AT_ReAddIndex;
 			newcmd->def = (Node *) stmt;
 			tab->subcmds[AT_PASS_OLD_INDEX] =
 				lappend(tab->subcmds[AT_PASS_OLD_INDEX], newcmd);
-					break;
 		}
-			case T_AlterTableStmt:
+		else if (IsA(stm, AlterTableStmt))
 		{
 			AlterTableStmt *stmt = (AlterTableStmt *) stm;
 			ListCell   *lcmd;
 
-					tab = ATGetQueueEntry(wqueue, rel);
 			foreach(lcmd, stmt->cmds)
 			{
 				AlterTableCmd *cmd = (AlterTableCmd *) lfirst(lcmd);
-						Constraint *con;
 
-						switch (cmd->subtype)
+				if (cmd->subtype == AT_AddIndex)
 				{
-							case AT_AddIndex:
 					Assert(IsA(cmd->def, IndexStmt));
+
 					if (!rewrite)
 						TryReuseIndex(get_constraint_index(oldId),
 									  (IndexStmt *) cmd->def);
+
 					cmd->subtype = AT_ReAddIndex;
 					tab->subcmds[AT_PASS_OLD_INDEX] =
 						lappend(tab->subcmds[AT_PASS_OLD_INDEX], cmd);
-								break;
-							case AT_AddConstraint:
+				}
+				else if (cmd->subtype == AT_AddConstraint)
+				{
+					Constraint *con;
+
 					Assert(IsA(cmd->def, Constraint));
+
 					con = (Constraint *) cmd->def;
 					con->old_pktable_oid = refRelId;
 					/* rewriting neither side of a FK */
@@ -8696,19 +8697,16 @@ ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId, char *cmd,
 					cmd->subtype = AT_ReAddConstraint;
 					tab->subcmds[AT_PASS_OLD_CONSTR] =
 						lappend(tab->subcmds[AT_PASS_OLD_CONSTR], cmd);
-								break;
-							default:
+				}
+				else
 					elog(ERROR, "unexpected statement type: %d",
 						 (int) cmd->subtype);
 			}
 		}
-					break;
-				}
-			default:
+		else
 			elog(ERROR, "unexpected statement type: %d",
 				 (int) nodeTag(stm));
 	}
-	}
 
 	relation_close(rel, NoLock);
 }