cmd/compile/internal/reflectdata: fix divide by zero for zero-size array elements

When generating equality signatures for arrays with zero-size ASPECIAL
elements (e.g., [3]struct{_ [0]float64}), the compiler crashed with
a divide by zero error when computing the loop unroll factor.

Skip comparison code generation for zero-size elements since they
need no comparison.

Fixes #77303

Change-Id: Ib432cfece22b1cb714de4f0a0b0d1a2d89bb0d33
Reviewed-on: https://go-review.googlesource.com/c/go/+/738841
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/compile/internal/reflectdata/alg.go

@@ -785,6 +785,9 @@ func (e *eqSigBuilder) build(t *types.Type) {
 			// The generated loops are kind of inefficient as well,
 			// so unroll the loop a bit.
 			const unrollSize = 32 // make loop body compare around this many bytes
+			if et.Size() == 0 {
+				break // zero-size elements need no comparison
+			}
 			unroll := max(1, unrollSize/et.Size())
 			// Do partial loops directly.
 			for n%unroll != 0 {

test/fixedbugs/issue77303.go

@@ -0,0 +1,19 @@
+// compile
+
+// Copyright 2026 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 77303: compiler crash on array of zero-size ASPECIAL elements.
+
+package p
+
+type zeroSizeSpecial struct {
+	_ [0]float64
+}
+
+var x [3]zeroSizeSpecial
+
+func f() bool {
+	return x == x
+}