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>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/reflect/value.go b/src/reflect/value.go
index 90edf8e31d..ecf9dd7bc8 100644
--- a/src/reflect/value.go
+++ b/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
 }