cmd/internal/gc: fix broken build

CL 7697 caused doasm failures on 386:

runtime/append_test.go:1: doasm: notfound ft=2 tt=20 00112 (runtime/iface_test.go:207)	CMPL	$0, BX 2 20

I think that this should be fixed in liblink,
but in the meantime, work around the problem
by instead generating CMPL BX, $0.

Change-Id: I9c572f8f15fc159507132cf4ace8d7a328a3eb4a
Reviewed-on: https://go-review.googlesource.com/7810Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/internal/gc/walk.go

@@ -886,7 +886,7 @@ func walkexpr(np **Node, init **NodeList) {
 			case fromKind == "I" && toKind == "E",
 				fromKind == "E" && toKind == "E":
 				tab := Nod(OITAB, from, nil)
-				fast = Nod(ONE, tab, nodnil())
+				fast = Nod(ONE, nodnil(), tab)
 			}
 			if fast != nil {
 				n = Nod(OAS, ok, fast)