8g: out of register bug

Fixes #806.

R=ken2
CC=golang-dev
https://golang.org/cl/1281042

test/fixedbugs/bug283.go

@@ -0,0 +1,19 @@
+// $G $D/$F.go || echo BUG: should compile
+
+// 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.
+
+// http://code.google.com/p/go/issues/detail?id=806
+// triggered out of registers on 8g
+
+package main
+
+type Point struct {
+	x int
+	y int
+}
+
+func dist(p0, p1 Point) float64 {
+	return float64((p0.x-p1.x)*(p0.x-p1.x) + (p0.y-p1.y)*(p0.y-p1.y))
+}