cgen: allow for const ( x = opt() ? )

vlib/regex/simple_const_regex_test.v

@@ -0,0 +1,17 @@
+import regex
+
+const (
+	a_or_b = regex.regex_opt('a|b') ?
+)
+
+fn f(s string) bool {
+	mut re := a_or_b
+	start, _ := re.match_string(s)
+	return start != -1
+}
+
+fn test_const_regex_works() {
+	assert f('a') == true
+	assert f('b') == true
+	assert f('c') == false
+}