Fix is_expression and is_pattern_expression parsing

[tamasvajk]

The below two is expressions produce different parse trees. The former is correctly parsed as is_expression and latter is incorrectly parsed as is_pattern_expression.

var x = y is int;
var x = y is A;

(compilation_unit [0, 0] - [1, 15]
  (global_statement [0, 0] - [0, 17]
    (local_declaration_statement [0, 0] - [0, 17]
      (variable_declaration [0, 0] - [0, 16]
        type: (implicit_type [0, 0] - [0, 3])
        (variable_declarator [0, 4] - [0, 16]
          (identifier [0, 4] - [0, 5])
          (equals_value_clause [0, 6] - [0, 16]
            (is_expression [0, 8] - [0, 16]
              left: (identifier [0, 8] - [0, 9])
              right: (predefined_type [0, 13] - [0, 16])))))))
  (global_statement [1, 0] - [1, 15]
    (local_declaration_statement [1, 0] - [1, 15]
      (variable_declaration [1, 0] - [1, 14]
        type: (implicit_type [1, 0] - [1, 3])
        (variable_declarator [1, 4] - [1, 14]
          (identifier [1, 4] - [1, 5])
          (equals_value_clause [1, 6] - [1, 14]
            (is_pattern_expression [1, 8] - [1, 14]
              expression: (identifier [1, 8] - [1, 9])
              pattern: (constant_pattern [1, 13] - [1, 14]
                (identifier [1, 13] - [1, 14])))))))))