runtime: make evacDst a top level type

This will reduce duplication when evacuate is specialized.

Change-Id: I34cdfb7103442d3e0ea908c970fb46334b86d5c4
Reviewed-on: https://go-review.googlesource.com/56934
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/hashmap.go

@@ -1021,6 +1021,14 @@ func bucketEvacuated(t *maptype, h *hmap, bucket uintptr) bool {
 	return evacuated(b)
 }
 
+// evacDst is an evacuation destination.
+type evacDst struct {
+	b *bmap          // current destination bucket
+	i int            // key/val index into b
+	k unsafe.Pointer // pointer to current key storage
+	v unsafe.Pointer // pointer to current value storage
+}
+
 func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 	b := (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
 	newbit := h.noldbuckets()
@@ -1028,14 +1036,6 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 		// TODO: reuse overflow buckets instead of using new ones, if there
 		// is no iterator using the old buckets.  (If !oldIterator.)
 
-		// evacDst is an evacuation destination.
-		type evacDst struct {
-			b *bmap          // current destination bucket
-			i int            // key/val index into b
-			k unsafe.Pointer // pointer to current key storage
-			v unsafe.Pointer // pointer to current value storage
-		}
-
 		// xy contains the x and y (low and high) evacuation destinations.
 		var xy [2]evacDst
 		x := &xy[0]