test: test handling of negative float constants

This used to panic when compiled by gccgo.

Updates #2876.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6100055

test/fixedbugs/bug434.go

+// run
+
+// Copyright 2012 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.
+
+// Test that typed and untyped negative zero floating point constants
+// are treated as equivalent to zero constants.
+
+package main
+
+import "math"
+
+const zero = 0.0
+
+func main() {
+	x := -zero
+	b := math.Float64bits(x)
+	if b != 0 {
+		panic(b)
+	}
+	x = -float64(zero)
+	b = math.Float64bits(x)
+	if b != 0 {
+		panic(b)
+	}
+	v := x
+	b = math.Float64bits(-v)
+	if b != 0x8000000000000000 {
+		panic(b)
+	}
+}