cmd/compile: checkwidth T when constructing *T

Without this, T can sneak through to the backend
with its width unknown.

Fixes #20174

Change-Id: I9b21e0e2641f75e360cc5e45dcb4eefe8255b675
Reviewed-on: https://go-review.googlesource.com/42175
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -498,6 +498,7 @@ OpSwitch:
 			ok |= Etype
 			n.Op = OTYPE
 			n.Type = types.NewPtr(l.Type)
+			checkwidth(l.Type) // ensure this gets dowidth'd for the backend
 			n.Left = nil
 			break OpSwitch
 		}

test/fixedbugs/issue20174.go

+// compile -c=2
+
+// 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.
+
+// Issue 20174: failure to typecheck contents of *T in the frontend.
+
+package p
+
+func f() {
+	_ = (*interface{})(nil) // interface{} here used to not have its width calculated going into backend
+	select {
+	case _ = <-make(chan interface {
+		M()
+	}, 1):
+	}
+}