cmd/8g: fix miscompilation due to BADWIDTH.

Fixes #3899.

R=rsc
CC=golang-dev, remy
https://golang.org/cl/6453084

src/cmd/8g/gsubr.c

@@ -1853,6 +1853,7 @@ naddr(Node *n, Addr *a, int canemitcode)
 		a->width = 0;
 		if(n->type != T) {
 			a->etype = simtype[n->type->etype];
+			dowidth(n->type);
 			a->width = n->type->width;
 			a->gotype = ngotype(n);
 		}

test/fixedbugs/bug450.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.
+
+// Issue 3899: 8g incorrectly thinks a variable is
+// "set but not used" and elides an assignment, causing
+// variables to end up with wrong data.
+//
+// The reason is a miscalculation of variable width.
+
+package main
+
+func bar(f func()) {
+	f()
+}
+
+func foo() {
+	f := func() {}
+	if f == nil {
+	}
+	bar(f)
+}
+
+func main() {
+	foo()
+}