cmd/compile: only SSA [0]T when T is SSA-able

Almost never happens in practice.
The compiler will generate reasonable code anyway,
since assignments involving [0]T never do any work.

Fixes #19696
Fixes #19671

Change-Id: I350d2e0c5bb326c4789c74a046ab0486b2cee49c
Reviewed-on: https://go-review.googlesource.com/38599
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -3294,10 +3294,7 @@ func canSSAType(t *Type) bool {
 		// We can't do larger arrays because dynamic indexing is
 		// not supported on SSA variables.
 		// TODO: allow if all indexes are constant.
-		if t.NumElem() == 0 {
-			return true
-		}
-		if t.NumElem() == 1 {
+		if t.NumElem() <= 1 {
 			return canSSAType(t.Elem())
 		}
 		return false

test/fixedbugs/issue19671.go

+// compile
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Used to crash when compiling assignments involving [0]T,
+// where T is not SSA-able.
+
+package a
+
+func f() {
+	var i int
+	arr := [0][2]int{}
+	arr[i][0] = 0
+}

test/fixedbugs/issue19696.go

+// compile
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Used to crash when compiling assignments involving [0]T,
+// where T is not SSA-able.
+
+package p
+
+type s struct {
+	a, b, c, d, e int
+}
+
+func f() {
+	var i int
+	arr := [0]s{}
+	arr[i].a++
+}