This improves performance for maps with a bucket size
(key+value*8 bytes) larger than 32 bytes and removes loading
a value from the maxElems array for smaller bucket sizes.

name                old time/op  new time/op  delta
MakeMap/[Byte]Byte  93.5ns ± 1%  91.8ns ± 1%  -1.83%  (p=0.000 n=10+10)
MakeMap/[Int]Int     134ns ± 1%   127ns ± 2%  -5.61%  (p=0.000 n=9+10)

Updates #21588

Change-Id: I53f77186769c4bd0f2b90f3c6c17df643b060e39
Reviewed-on: https://go-review.googlesource.com/c/143797
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>