This improves performance for e.g. 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  95.5ns ± 1%  94.7ns ± 1%  -0.78%  (p=0.013 n=9+9)
MakeMap/[Int]Int     128ns ± 0%   121ns ± 2%  -5.63%  (p=0.000 n=6+10)

Updates #21588

Change-Id: I7d9eb7d49150c399c15dcab675e24bc97ff97852
Reviewed-on: https://go-review.googlesource.com/c/143997Reviewed-by: Keith Randall <khr@golang.org>