Only compute the number of maximum allowed elements per slice once.

name         old time/op  new time/op  delta
MakeSlice-2  55.5ns ± 1%  45.6ns ± 2%  -17.88%  (p=0.000 n=99+100)

Change-Id: I951feffda5d11910a75e55d7e978d306d14da2c5
Reviewed-on: https://go-review.googlesource.com/21801
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>