cmd/compile: set OpLoad argument type interface{} correctly

CL 271906 allows loading single field of typed-interface{} OpIData, but
it does not update the corresponding selector type. So the generated
OpLoad has the named type instead, prevent it from being lowered by
lower pass.

Fixes #42784

Change-Id: Idf32e4f711731be09d508dd712b60bc8c58309bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/272466
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: Cherry Zhang <cherryyz@google.com>

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

@@ -250,6 +250,9 @@ func expandCalls(f *Func) {
 			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())
 			}
+			if selector.Type.IsEmptyInterface() {
+				selector.Type = typ.BytePtr
+			}
 			leaf.copyOf(selector)
 			for _, s := range namedSelects[selector] {
 				locs = append(locs, f.Names[s.locIndex])

test/fixedbugs/issue42784.go

@@ -0,0 +1,26 @@
+// 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 set OpLoad argument type interface{}
+
+package p
+
+type iface interface {
+	m()
+}
+
+type it interface{}
+
+type makeIface func() iface
+
+func f() {
+	var im makeIface
+	e := im().(it)
+	g(e)
+}
+
+//go:noinline
+func g(i it) {}