runtime: split advanceEvacuationMark from evacuate

Minor refactoring. This is a step towards specializing evacuate
for mapfast key types.

Change-Id: Icffe2759b7d38e5c008d03941918d5a912ce62f6
Reviewed-on: https://go-review.googlesource.com/56933
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/hashmap.go

@@ -1138,8 +1138,12 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 		}
 	}
 
-	// Advance evacuation mark
 	if oldbucket == h.nevacuate {
+		advanceEvacuationMark(h, t, newbit)
+	}
+}
+
+func advanceEvacuationMark(h *hmap, t *maptype, newbit uintptr) {
 	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.
@@ -1161,7 +1165,6 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 		}
 		h.flags &^= sameSizeGrow
 	}
-	}
 }
 
 func ismapkey(t *_type) bool {