Use PM_BINDING_POWER_COMPOSITION for the conditional predicate floor

The `+ 1` form is meant to express associativity, not a binding-power
floor; use the named constant instead, matching while/until conditions.

Author: kai-matsudate

src/prism.c

@@ -15486,7 +15486,7 @@ parse_conditional(pm_parser_t *parser, pm_context_t context, size_t opening_newl
     pm_token_t keyword = parser->previous;
     pm_token_t then_keyword = { 0 };
 
-    pm_node_t *predicate = parse_predicate(parser, PM_BINDING_POWER_MODIFIER + 1, context, &then_keyword, (uint16_t) (depth + 1));
+    pm_node_t *predicate = parse_predicate(parser, PM_BINDING_POWER_COMPOSITION, context, &then_keyword, (uint16_t) (depth + 1));
     pm_statements_node_t *statements = NULL;
 
     if (!match3(parser, PM_TOKEN_KEYWORD_ELSIF, PM_TOKEN_KEYWORD_ELSE, PM_TOKEN_KEYWORD_END)) {
@@ -15524,7 +15524,7 @@ parse_conditional(pm_parser_t *parser, pm_context_t context, size_t opening_newl
             pm_token_t elsif_keyword = parser->current;
             parser_lex(parser);
 
-            pm_node_t *predicate = parse_predicate(parser, PM_BINDING_POWER_MODIFIER + 1, PM_CONTEXT_ELSIF, &then_keyword, (uint16_t) (depth + 1));
+            pm_node_t *predicate = parse_predicate(parser, PM_BINDING_POWER_COMPOSITION, PM_CONTEXT_ELSIF, &then_keyword, (uint16_t) (depth + 1));
             pm_accepts_block_stack_push(parser, true);
 
             pm_statements_node_t *statements = parse_statements(parser, PM_CONTEXT_ELSIF, (uint16_t) (depth + 1));