runtime: mask a bounded slice access in hashmap evacuate

Shaves a few instructions off.

Change-Id: I39f1b01ae7e770d632d5e77a6aa4b5a1f123b41a
Reviewed-on: https://go-review.googlesource.com/55090
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

@@ -1129,7 +1129,7 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 					dst.k = add(unsafe.Pointer(dst.b), dataOffset)
 					dst.v = add(dst.k, bucketCnt*uintptr(t.keysize))
 				}
-				dst.b.tophash[dst.i] = top
+				dst.b.tophash[dst.i&(bucketCnt-1)] = top // mask dst.i as an optimization, to avoid a bounds check
 				if t.indirectkey {
 					*(*unsafe.Pointer)(dst.k) = k2 // copy pointer
 				} else {