runtime: if key type is reflexive, don't call equal(k, k)

Most types are reflexive (k == k for all k of type t), so don't
bother calling equal(k, k) when the key type is reflexive.

Change-Id: Ia716b4198b8b298687843b94b878dbc5e8fc2c65
Reviewed-on: https://go-review.googlesource.com/1480
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/type.go

@@ -73,6 +73,7 @@ type maptype struct {
 	valuesize     uint8  // size of value slot
 	indirectvalue bool   // store ptr to value instead of value itself
 	bucketsize    uint16 // size of bucket
+	reflexivekey  bool   // true if k==k for all keys
 }
 
 type chantype struct {