cmd/compile: preserve type information in inrange

Fixes #17551.

Change-Id: I84b7d82654cda3559c119aa56b07f30d0d224865
Reviewed-on: https://go-review.googlesource.com/31857
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -3561,7 +3561,10 @@ func walkinrange(n *Node, init *Nodes) *Node {
 	cmp.Lineno = n.Lineno
 	cmp = addinit(cmp, l.Ninit.Slice())
 	cmp = addinit(cmp, r.Ninit.Slice())
+	// Typecheck the AST rooted at cmp...
 	cmp = typecheck(cmp, Erv)
+	// ...but then reset cmp's type to match n's type.
+	cmp.Type = n.Type
 	cmp = walkexpr(cmp, init)
 	return cmp
 }

test/fixedbugs/issue17551.go

+// compile
+
+// 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.
+
+// Issue 17551: inrange optimization failed to preserve type information.
+
+package main
+
+import "fmt"
+
+func main() {
+	_, x := X()
+	fmt.Printf("x = %v\n", x)
+}
+
+func X() (i int, ok bool) {
+	ii := int(1)
+	return ii, 0 <= ii && ii <= 0x7fffffff
+}