internal/impl: remove unnecessary atomic access for non-lazy lists

Only lazy fields need to be set/read atomically, and the
fieldInfoForMessageListOpaqueNoPresence function is used for non-lazy fields.
(In fact, protoc currently does not allow [lazy = true] on repeated fields.)

I was not able to measure a performance difference either way.
(See Google-internal CL 759558617 for test results.)

Change-Id: Ie4c08a1345b62ecbe0f005d6cc97be35a8fe4daf
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/674055
Reviewed-by: Lasse Folger <lassefolger@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cassondra Foesch <cfoesch@gmail.com>

internal/impl/message_opaque.go

diff --git a/internal/impl/message_opaque.go b/internal/impl/message_opaque.go
index 63a731b..5a439da 100644
--- a/internal/impl/message_opaque.go
+++ b/internal/impl/message_opaque.go
@@ -344,17 +344,15 @@
 			if p.IsNil() {
 				return false
 			}
-			sp := p.Apply(fieldOffset).AtomicGetPointer()
-			if sp.IsNil() {
+			rv := p.Apply(fieldOffset).AsValueOf(fs.Type).Elem()
+			if rv.IsNil() {
 				return false
 			}
-			rv := sp.AsValueOf(fs.Type.Elem())
 			return rv.Elem().Len() > 0
 		},
 		clear: func(p pointer) {
-			sp := p.Apply(fieldOffset).AtomicGetPointer()
-			if !sp.IsNil() {
-				rv := sp.AsValueOf(fs.Type.Elem())
+			rv := p.Apply(fieldOffset).AsValueOf(fs.Type).Elem()
+			if !rv.IsNil() {
 				rv.Elem().Set(reflect.Zero(rv.Type().Elem()))
 			}
 		},
@@ -362,11 +360,10 @@
 			if p.IsNil() {
 				return conv.Zero()
 			}
-			sp := p.Apply(fieldOffset).AtomicGetPointer()
-			if sp.IsNil() {
+			rv := p.Apply(fieldOffset).AsValueOf(fs.Type).Elem()
+			if rv.IsNil() {
 				return conv.Zero()
 			}
-			rv := sp.AsValueOf(fs.Type.Elem())
 			if rv.Elem().Len() == 0 {
 				return conv.Zero()
 			}