cmd/compile: fix live variable reuse in orderstmt

The call "poptemp(t, order)" at line 906 should match up with the
assignment "t := marktemp(order)" at line 770, so use a new temporary
variable for stripping the ODCL nodes from a "case x := <-ch" node's
Ninit list.

Fixes #13469.
Passes toolstash/buildall.

Change-Id: Ia7eabd40c79cfdcb83df00b6fbd0954e0c44c5c7
Reviewed-on: https://go-review.googlesource.com/17393Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>

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

@@ -796,14 +796,14 @@ func orderstmt(n *Node, order *Order) {
 				// Delete the ODCL nodes here and recreate them inside the body below.
 				case OSELRECV, OSELRECV2:
 					if r.Colas {
-						t = r.Ninit
-						if t != nil && t.N.Op == ODCL && t.N.Left == r.Left {
-							t = t.Next
+						init := r.Ninit
+						if init != nil && init.N.Op == ODCL && init.N.Left == r.Left {
+							init = init.Next
 						}
-						if t != nil && t.N.Op == ODCL && r.List != nil && t.N.Left == r.List.N {
-							t = t.Next
+						if init != nil && init.N.Op == ODCL && r.List != nil && init.N.Left == r.List.N {
+							init = init.Next
 						}
-						if t == nil {
+						if init == nil {
 							r.Ninit = nil
 						}
 					}