reflect: add test of maps with keys larger than key size

This finds the bug fixed in CL 630279.

reflect mutates the SwissMapType of a map[unsafe.Pointer]unsafe.Pointer,
which happened to already have the correct GroupSize for all of the maps
used in the reflect tests.

For #54766.

Change-Id: If4428e1e799598e7512edceb3cefb2ad00cfa712
Reviewed-on: https://go-review.googlesource.com/c/go/+/630676
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

src/reflect/all_test.go

@@ -6309,6 +6309,32 @@ func TestMapOfGCKeys(t *testing.T) {
 	}
 }
 
+// Test assignment and access to a map with keys larger than word size.
+func TestMapOfGCBigKey(t *testing.T) {
+	type KV struct {
+		i int64
+		j int64
+	}
+
+	kvTyp := TypeFor[KV]()
+	mt := MapOf(kvTyp, kvTyp)
+
+	const n = 100
+	m := MakeMap(mt)
+	for i := 0; i < n; i++ {
+		kv := KV{int64(i), int64(i+1)}
+		m.SetMapIndex(ValueOf(kv), ValueOf(kv))
+	}
+
+	for i := 0; i < n; i++ {
+		kv := KV{int64(i), int64(i+1)}
+		elem := m.MapIndex(ValueOf(kv)).Interface().(KV)
+		if elem != kv {
+			t.Errorf("lost m[%v] = %v, want %v", kv, elem, kv)
+		}
+	}
+}
+
 func TestMapOfGCValues(t *testing.T) {
 	type T *uintptr
 	tt := TypeOf(T(nil))