cmd/compile: don't export dead code in inlineable fuctions

CL 37499 allows inlining more functions by ignoring dead code.
However, that dead code can contain non-exportable constructs.
Teach the exporter not to export dead code.

Fixes #19679 

Change-Id: Idb1d3794053514544b6f1035d29262aa6683e1e7
Reviewed-on: https://go-review.googlesource.com/38601
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -1493,8 +1493,22 @@ func (p *exporter) stmt(n *Node) {
 		p.pos(n)
 		p.stmtList(n.Ninit)
 		p.expr(n.Left)
-		p.stmtList(n.Nbody)
-		p.stmtList(n.Rlist)
+		nbody := n.Nbody
+		rlist := n.Rlist
+		if Isconst(n.Left, CTBOOL) {
+			// if false { ... } or if true { ... }
+			// Only export the taken branch.
+			// This is more efficient,
+			// and avoids trying to export
+			// un-exportable nodes.
+			if n.Left.Bool() {
+				rlist = Nodes{}
+			} else {
+				nbody = Nodes{}
+			}
+		}
+		p.stmtList(nbody)
+		p.stmtList(rlist)
 
 	case OFOR:
 		p.op(OFOR)

test/fixedbugs/issue19679.go

@@ -0,0 +1,38 @@
+// compile
+
+// Copyright 2017 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.
+
+// Used to crash when a type switch was present in dead code
+// in an inlineable function.
+
+package p
+
+func Then() {
+	var i interface{}
+	if false {
+		switch i.(type) {
+		}
+	}
+}
+
+func Else() {
+	var i interface{}
+	if true {
+		_ = i
+	} else {
+		switch i.(type) {
+		}
+	}
+}
+
+func Switch() {
+	var i interface{}
+	switch 5 {
+	case 3:
+		switch i.(type) {
+		}
+	case 5:
+	}
+}