cmd/compile: fix register size for ODOTPTR result

The result of ODOTPTR, as well as a bunch of other ops,
should be the type of the result, not always a pointer type.

This fixes an amd64p32 bug where we were incorrectly truncating
a 64-bit slice index to 32 bits, and then barfing on a weird
load-64-bits-but-then-truncate-to-32-bits op that doesn't exist.

Fixes #15252

Change-Id: Ie62f4315fffd79f233e5449324ccc0879f5ac343
Reviewed-on: https://go-review.googlesource.com/22094
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -946,7 +946,7 @@ func Cgenr(n *Node, a *Node, res *Node) {
 		OCALLINTER:
 		var n1 Node
 		Igen(n, &n1, res)
-		Regalloc(a, Types[Tptr], &n1)
+		Regalloc(a, n.Type, &n1)
 		Thearch.Gmove(&n1, a)
 		Regfree(&n1)
 

test/fixedbugs/issue15252.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.
+
+// This test makes sure that we use all 64 bits of an
+// index, even on 32 bit machines.  It also tests that nacl
+// can compile 64 bit indexes loaded from ODOTPTR properly.
+
+package main
+
+type T struct {
+	i int64
+}
+
+func f(t *T) byte {
+	b := [2]byte{3, 4}
+	return b[t.i]
+}
+
+func main() {
+	t := &T{0x100000001}
+	defer func() {
+		r := recover()
+		if r == nil {
+			panic("panic wasn't recoverable")
+		}
+	}()
+	f(t)
+	panic("index didn't panic")
+}