test/fixedbugs: forgotten test cases for issues 3907 and 4156.

Update #3907.
Update #4156.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/6595044
diff --git a/test/fixedbugs/bug456.go b/test/fixedbugs/bug456.go
new file mode 100644
index 0000000..064e1aa
--- /dev/null
+++ b/test/fixedbugs/bug456.go
@@ -0,0 +1,26 @@
+// 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 3907: out of fixed registers in nested byte multiply.
+// Used to happen with both 6g and 8g.
+
+package main
+
+func F(a, b, c, d uint8) uint8 {
+	return a * (b * (c * (d *
+		(a * (b * (c * (d *
+			(a * (b * (c * (d *
+				a * (b * (c * d)))))))))))))
+}
+
+func main() {
+	var a, b, c, d uint8 = 1, 1, 1, 1
+	x := F(a, b, c, d)
+	if x != 1 {
+		println(x)
+		panic("x != 1")
+	}
+}