cmd/6g, cmd/8g: prevent constant propagation of non-constant LEA.

Fixes #5809.

R=golang-dev, dave, rsc, nigeltao
CC=golang-dev
https://golang.org/cl/10785043

src/cmd/6g/peep.c

@@ -154,6 +154,7 @@ peep(void)
 		case ALEAQ:
 			if(regtyp(&p->to))
 			if(p->from.sym != S)
+			if(p->from.index == D_NONE || p->from.index == D_CONST)
 				conprop(r);
 			break;
 

src/cmd/8g/peep.c

@@ -147,6 +147,7 @@ peep(void)
 		case ALEAL:
 			if(regtyp(&p->to))
 			if(p->from.sym != S)
+			if(p->from.index == D_NONE || p->from.index == D_CONST)
 				conprop(r);
 			break;
 

test/fixedbugs/issue5809.go

+// run
+
+// Copyright 2013 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 5809: 6g and 8g attempted to constant propagate indexed LEA
+
+package main
+
+import "fmt"
+
+func main() {
+	const d16 = "0123456789ABCDEF"
+	k := 0x1234
+	var x [4]byte
+	
+	x[0] = d16[k>>12&0xf]
+	x[1] = d16[k>>8&0xf]
+	x[2] = d16[k>>4&0xf]
+	x[3] = d16[k&0xf]
+	
+	if x != [4]byte{'1','2','3','4'} {
+		fmt.Println(x)
+		panic("x != [4]byte{'1','2','3','4'}")
+	}
+}