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>

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{}
+}