Support optional else-branch for if-then-elif-end (#2598)

src/parser.y

@@ -367,9 +367,6 @@ Term "as" Patterns '|' Exp {
 "if" Exp "then" Exp ElseBody {
   $$ = gen_cond($2, $4, $5);
 } |
-"if" Exp "then" Exp "end" {
-  $$ = gen_cond($2, $4, gen_noop());
-} |
 "if" Exp "then" error {
   FAIL(@$, "Possibly unterminated 'if' statement");
   $$ = $2;
@@ -625,6 +622,9 @@ ElseBody:
 } |
 "else" Exp "end" {
   $$ = $2;
+} |
+"end" {
+  $$ = gen_noop();
 }
 
 ExpD: