runtime: tiny refactor in evacuate

Since oldbucket == h.nevacuate, we can just increment h.nevacuate here.
This removes oldbucket from scope, which will be useful shortly.

Change-Id: I70f81ec3995f17845ebf5d77ccd20ea4338f23e6
Reviewed-on: https://go-review.googlesource.com/56932
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

@@ -1140,7 +1140,7 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 
 	// Advance evacuation mark
 	if oldbucket == h.nevacuate {
-		h.nevacuate = oldbucket + 1
+		h.nevacuate++
 		// Experiments suggest that 1024 is overkill by at least an order of magnitude.
 		// Put it in there as a safeguard anyway, to ensure O(1) behavior.
 		stop := h.nevacuate + 1024