8g: out of register bug fix

Fixes #868.

R=ken2
CC=golang-dev
https://golang.org/cl/1695049

src/cmd/8g/ggen.c

@@ -672,8 +672,10 @@ cgen_shift(int op, Node *nl, Node *nr, Node *res)
 	a = optoas(op, nl->type);
 
 	if(nr->op == OLITERAL) {
+		tempname(&n2, nl->type);
+		cgen(nl, &n2);
 		regalloc(&n1, nl->type, res);
-		cgen(nl, &n1);
+		gmove(&n2, &n1);
 		sc = mpgetfix(nr->val.u.xval);
 		if(sc >= nl->type->width*8) {
 			// large shift gets 2 shifts by width

test/fixedbugs/bug287.go

+// errchk $G $D/$F.go
+
+// Copyright 2010 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 die dividing by zero; issue 879.
+
+package main
+
+var mult [3][...]byte = [3][5]byte{}	// ERROR "\.\.\."

test/fixedbugs/bug288.go

+// $G $D/$F.go
+
+// Copyright 2010 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 run out of registers on 8g.  Issue 868.
+
+package main
+
+func main() {
+	var r uint32
+	var buf [4]byte
+	a := buf[0:4]
+	r = (((((uint32(a[3]) << 8) | uint32(a[2])) << 8) |
+		uint32(a[1])) << 8) | uint32(a[0])
+	_ = r
+}