reflect: avoid stack copies of hiter

Use a pointer reciever to avoid copying the hiter struct when
checking if it is intialized.

Found through profiling that showed reflect map iteration spending
a good amount of time in duffcopy.

This change will also help other MapIter methods checking hiter struct
initialization like Value() and Key().

name            old time/op  new time/op  delta
MapIterNext-12  97.9ns ± 4%  83.8ns ± 2%  -14.37%  (p=0.000 n=10+10)

Change-Id: I73ab964fa28061ee7e6d5c663a85048bd2e0274e
Reviewed-on: https://go-review.googlesource.com/c/go/+/360254
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Martin Möhrmann <martin@golang.org>

src/reflect/all_test.go

@@ -7568,6 +7568,16 @@ func TestMapIterNext(t *testing.T) {
 	}
 }
 
+func BenchmarkMapIterNext(b *testing.B) {
+	m := ValueOf(map[string]int{"a": 0, "b": 1, "c": 2, "d": 3})
+	it := m.MapRange()
+	for i := 0; i < b.N; i++ {
+		for it.Next() {
+		}
+		it.Reset(m)
+	}
+}
+
 func TestMapIterDelete0(t *testing.T) {
 	// Delete all elements before first iteration.
 	m := map[string]int{"one": 1, "two": 2, "three": 3}

src/reflect/value.go

@@ -1665,7 +1665,7 @@ type hiter struct {
 	checkBucket uintptr
 }
 
-func (h hiter) initialized() bool {
+func (h *hiter) initialized() bool {
 	return h.t != nil
 }