test: Add test which causes incorrect error from gccgo.

R=gri
CC=golang-dev
https://golang.org/cl/2220048

test/fixedbugs/bug307.go

+// $G $D/$F.go
+
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Valid program, gccgo reported an error.
+// bug307.go:14:6: error: cmplx arguments must have identical types
+
+package main
+
+func main() {
+	var f float64
+	_ = cmplx(1 / f, 0)
+}