cmd/compile: add 0-sized-value simplification to copyelim

The problem was caused by faulty handling of unSSA-able
operations on zero-sized data in expand calls, but there
is no point to operations on zero-sized data.  This CL adds
a simplify step to the first place in SSA where all values
are processed and replaces anything producing a 0-sized
struct/array with the corresponding Struct/Array Make0
operation (of the appropriate type).

I attempted not generating them in ssagen, but that was a
larger change, and also had bugs. This is simple and obvious.
The only question is whether it would be worthwhile to do it
earlier (in numberlines or phielem).

Fixes #65808.

Change-Id: I0a596b3d272798015e7bb6b1a20411241759fe0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/568258
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -11,6 +11,17 @@ func copyelim(f *Func) {
 	// of OpCopy) is a copy.
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
+
+			// This is an early place in SSA where all values are examined.
+			// Rewrite all 0-sized Go values to remove accessors, dereferences, loads, etc.
+			if t := v.Type; (t.IsStruct() || t.IsArray()) && t.Size() == 0 {
+				if t.IsStruct() {
+					v.reset(OpStructMake0)
+				} else {
+					v.reset(OpArrayMake0)
+				}
+			}
+
 			copyelimValue(v)
 		}
 	}

test/fixedbugs/issue65808.go

@@ -0,0 +1,30 @@
+// compile
+
+// Copyright 2024 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 main
+
+package main
+
+type Stringer interface {
+	String() string
+}
+
+type (
+	stringer  struct{}
+	stringers [2]stringer
+	foo       struct {
+		stringers
+	}
+)
+
+func (stringer) String() string  { return "" }
+func toString(s Stringer) string { return s.String() }
+
+func (v stringers) toStrings() []string {
+	return []string{toString(v[0]), toString(v[1])}
+}
+
+func main() {
+	_ = stringers{}
+}