cmd/compile: fix inlining of switch issue

The issue was seen when inlining an exported function that contained
a fallthrough statement.

Fixes #15071

Change-Id: I1e8215ad49d57673dba7e8f8bd2ed8ad290dc452
Reviewed-on: https://go-review.googlesource.com/21452Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/compile/internal/gc/fmt.go

@@ -189,6 +189,7 @@ var goopnames = []string{
 	OSUB:      "-",
 	OSWITCH:   "switch",
 	OXOR:      "^",
+	OXFALL:    "fallthrough",
 }
 
 // Fmt "%O":  Node opcodes

test/fixedbugs/issue15071.dir/exp/exp.go

+// Copyright 2016 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.
+
+package exp
+
+func Exported(x int) int {
+	return inlined(x)
+}
+
+func inlined(x int) int {
+	y := 0
+	switch {
+	case x > 0:
+		y += 5
+		return 0 + y
+	case x < 1:
+		y += 6
+		fallthrough
+	default:
+		y += 7
+		return 2 + y
+	}
+}

test/fixedbugs/issue15071.dir/main.go

+// run
+
+// Copyright 2016 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.
+
+package main
+
+import "os"
+import "./exp"
+
+func main() {
+	_ = exp.Exported(len(os.Args))
+}