cmd/compile: pass stack allocated bucket to makemap inside hmap

cmd/compile: pass stack allocated bucket to makemap inside hmap

name         old time/op    new time/op    delta
NewEmptyMap    53.2ns ± 7%    48.0ns ± 5%  -9.77%  (p=0.000 n=20+20)
NewSmallMap     111ns ± 1%     106ns ± 2%  -3.78%  (p=0.000 n=20+19)

Change-Id: I979d21ab16eae9f6893873becca517db57e054b5
Reviewed-on: https://go-review.googlesource.com/56290
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>