cmd/compile: change dead code into assert

After fixing #13587, I noticed that the "OAS2FUNC in disguise" block
looked like it probably needed write barriers too.  However, testing
revealed the multi-value "return f()" case was already being handled
correctly.

It turns out this block is dead code due to "return f()" already being
transformed into "t1, t2, ..., tN := f(); return t1, t2, ..., tN" by
orderstmt when f is a multi-valued function.

Updates #13587.

Change-Id: Icde46dccc55beda2ea5fd5fcafc9aae26cec1552
Reviewed-on: https://go-review.googlesource.com/17759
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -299,6 +299,12 @@ func walkstmt(np **Node) {
 				}
 			}
 
+			if got, want := count(n.List), count(rl); got != want {
+				// order should have rewritten multi-value function calls
+				// with explicit OAS2FUNC nodes.
+				Fatalf("expected %v return arguments, have %v", want, got)
+			}
+
 			if samelist(rl, n.List) {
 				// special return in disguise
 				n.List = nil
@@ -306,17 +312,6 @@ func walkstmt(np **Node) {
 				break
 			}
 
-			if count(n.List) == 1 && count(rl) > 1 {
-				// OAS2FUNC in disguise
-				f := n.List.N
-
-				if f.Op != OCALLFUNC && f.Op != OCALLMETH && f.Op != OCALLINTER {
-					Fatalf("expected return of call, have %v", f)
-				}
-				n.List = concat(list1(f), ascompatet(n.Op, rl, &f.Type, 0, &n.Ninit))
-				break
-			}
-
 			// move function calls out, to make reorder3's job easier.
 			walkexprlistsafe(n.List, &n.Ninit)