cmd/compile: properly handle map assignments for OAS2DOTTYPE

The boolean destination in an OAS2DOTTYPE expression craps out during
compilation when trying to assign to a map entry because, unlike slice entries,
map entries are not directly addressable in memory. The solution is to
properly order the boolean destination node so that map entries are set
via autotmp variables.

Fixes #14678

Change-Id: If344e8f232b5bdac1b53c0f0d21eeb43ab17d3de
Reviewed-on: https://go-review.googlesource.com/22833
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -569,18 +569,24 @@ func orderstmt(n *Node, order *Order) {
 
 		orderexprlist(n.List, order)
 		n.Rlist.First().Left = orderexpr(n.Rlist.First().Left, order, nil) // i in i.(T)
-		if isblank(n.List.First()) {
-			order.out = append(order.out, n)
-		} else {
-			typ := n.Rlist.First().Type
-			tmp1 := ordertemp(typ, order, haspointers(typ))
-			order.out = append(order.out, n)
-			r := Nod(OAS, n.List.First(), tmp1)
-			r = typecheck(r, Etop)
-			ordermapassign(r, order)
-			n.List.Set([]*Node{tmp1, n.List.Second()})
-		}
 
+		results := n.List.Slice()
+		var assigns [2]*Node
+
+		for r, res := range results {
+			if !isblank(res) {
+				results[r] = ordertemp(res.Type, order, haspointers(res.Type))
+				assigns[r] = Nod(OAS, res, results[r])
+			}
+		}
+		order.out = append(order.out, n)
+
+		for _, assign := range assigns {
+			if assign != nil {
+				assign = typecheck(assign, Etop)
+				ordermapassign(assign, order)
+			}
+		}
 		cleantemp(t, order)
 
 		// Special: use temporary variables to hold result,