cmd/compile: check order temp has correct type

Followon from CL 140306

Change-Id: Ic71033d2301105b15b60645d895a076107f44a2e
Reviewed-on: https://go-review.googlesource.com/c/142178
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -833,7 +833,9 @@ func slicelit(ctxt initContext, n *Node, var_ *Node, init *Nodes) {
 	var a *Node
 	if x := prealloc[n]; x != nil {
 		// temp allocated during order.go for dddarg
-		x.Type = t
+		if !eqtype(t, x.Type) {
+			panic("dotdotdot base type does not match order's assigned type")
+		}
 
 		if vstat == nil {
 			a = nod(OAS, x, nil)