cmd/compile: allow loading single field of typed-interface{} OpIData

Same reason as CL 270057, but for OpLoad.

Fixes #42727

Change-Id: Iebb1a8110f29427a0aed3b5e3e84f0540de3d1b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/271906
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -247,7 +247,7 @@ func expandCalls(f *Func) {
 			// i.e., the struct select is generated and remains in because it is not applied to an actual structure.
 			// The OpLoad was created to load the single field of the IData
 			// This case removes that StructSelect.
-			if leafType != selector.Type {
+			if leafType != selector.Type && !selector.Type.IsEmptyInterface() { // empty interface for #42727
 				f.Fatalf("Unexpected Load as selector, leaf=%s, selector=%s\n", leaf.LongString(), selector.LongString())
 			}
 			leaf.copyOf(selector)

test/fixedbugs/issue42727.go

@@ -0,0 +1,23 @@
+// compile
+
+// Copyright 2020 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.
+
+// Ensure that late expansion correctly handles an OpLoad with type interface{}
+
+package p
+
+type iface interface {
+	m()
+}
+
+type it interface{}
+
+type makeIface func() iface
+
+func f() {
+	var im makeIface
+	e := im().(it)
+	_ = &e
+}