Improve performance by reducing unnecessary function calls

Benchmarks:

Tme    old time/op  new time/op  delta
Cosh-8   229ns ± 0%   138ns ± 0%  -39.74%  (p=0.008 n=5+5)
Sinh-8   231ns ± 0%   139ns ± 0%  -39.83%  (p=0.008 n=5+5)

Change-Id: Icab5485849bbfaafca8429d06b67c558101f4f3c
Reviewed-on: https://go-review.googlesource.com/85477Reviewed-by: Robert Griesemer <gri@golang.org>