Make parseNodeString() C idiom compatible with Visual Studio 2015.

Between v15 and now, this function's "else if" chain grew from 252 lines
to 592 lines, exceeding a compiler limit that manifests as "fatal error
C1026: parser stack overflow, program too complex (compiling source file
src/backend/nodes/readfuncs.c)".  Use "if (...)  return ...;" instead.

Reviewed by Tom Lane, Peter Eisentraut and Michael Paquier.  Not all
reviewers endorse this.

Discussion: https://postgr.es/m/20230607185458.GA1334487@rfd.leadboat.com

src/backend/nodes/gen_node_support.pl

@@ -924,9 +924,9 @@ foreach my $n (@node_types)
 	  . "\t\t\t\t_out${n}(str, obj);\n"
 	  . "\t\t\t\tbreak;\n";
 
-	print $rfs "\telse if (MATCH(\"$N\", "
+	print $rfs "\tif (MATCH(\"$N\", "
 	  . length($N) . "))\n"
-	  . "\t\treturn_value = _read${n}();\n"
+	  . "\t\treturn (Node *) _read${n}();\n"
 	  unless $no_read;
 
 	next if elem $n, @custom_read_write;

src/backend/nodes/readfuncs.c

@@ -697,8 +697,6 @@ _readExtensibleNode(void)
 Node *
 parseNodeString(void)
 {
-	void	   *return_value;
-
 	READ_TEMP_LOCALS();
 
 	/* Guard against stack overflow due to overly complex expressions */
@@ -709,16 +707,10 @@ parseNodeString(void)
 #define MATCH(tokname, namelen) \
 	(length == namelen && memcmp(token, tokname, namelen) == 0)
 
-	if (false)
-		;
 #include "readfuncs.switch.c"
-	else
-	{
-		elog(ERROR, "badly formatted node string \"%.32s\"...", token);
-		return_value = NULL;	/* keep compiler quiet */
-	}
 
-	return (Node *) return_value;
+	elog(ERROR, "badly formatted node string \"%.32s\"...", token);
+	return NULL;				/* keep compiler quiet */
 }