cmd/compile: check SSAability in handling of INDEX of 1-element array

SSA can handle 1-element array, but only when the element type
is SSAable. When building SSA for INDEX of 1-element array, we
did not check the element type is SSAable. And when it's not,
it resulted in an unhandled SSA op.

Fixes #26120.

Change-Id: Id709996b5d9d90212f6c56d3f27eed320a4d8360
Reviewed-on: https://go-review.googlesource.com/121496
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -2170,8 +2170,9 @@ func (s *state) expr(n *Node) *ssa.Value {
 			p := s.addr(n, false)
 			return s.load(n.Left.Type.Elem(), p)
 		case n.Left.Type.IsArray():
-			if bound := n.Left.Type.NumElem(); bound <= 1 {
+			if canSSAType(n.Left.Type) {
 				// SSA can handle arrays of length at most 1.
+				bound := n.Left.Type.NumElem()
 				a := s.expr(n.Left)
 				i := s.expr(n.Right)
 				if bound == 0 {

test/fixedbugs/issue26120.go

@@ -0,0 +1,23 @@
+// compile
+
+// Copyright 2018 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.
+
+// Issue 26120: INDEX of 1-element but non-SSAable array
+// is mishandled when building SSA.
+
+package p
+
+type T [1]struct {
+	f    []int
+	i, j int
+}
+
+func F() {
+	var v T
+	f := func() T {
+		return v
+	}
+	_ = []int{}[f()[0].i]
+}