cmd/compile: fix compiler crash on recursive types

By setting both a valid size and alignment for broken recursive types,
we can appease some more safety checks and prevent compiler crashes.

Fixes #21882.

Change-Id: Ibaa137d8aa2c2a9d521462f144d7016c4abfd6e7
Reviewed-on: https://go-review.googlesource.com/64430
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -176,6 +176,7 @@ func dowidth(t *types.Type) {
 		}
 
 		t.Width = 0
+		t.Align = 1
 		return
 	}
 

test/fixedbugs/issue21882.go

+// errorcheck
+
+// 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.
+
+package p
+
+type T [2]T // ERROR "invalid recursive type"