math: optimize sinh and cosh
math: optimize sinh and cosh

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/85477
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

nawawi/go,cloudflare/go,akutz/go,nawawi/go,akutz/go,codestation/go,akutz/go,codestation/go,cloudflare/go,josharian/go,nawawi/go,Cofyc/go,Cofyc/go,cloudflare/go,josharian/go,josharian/go,nawawi/go,golang/go,golang/go,golang/go,golang/go,Cofyc/go,Cofyc/go,akutz/go,nawawi/go,golang/go,CAFxX/go,Cofyc/go,codestation/go,golang/go,codestation/go,golang/go,cloudflare/go,codestation/go,CAFxX/go,Cofyc/go,josharian/go,akutz/go,cloudflare/go,codestation/go,cloudflare/go,Cofyc/go,cloudflare/go,codestation/go,CAFxX/go,CAFxX/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,golang/go,josharian/go,nawawi/go,nawawi/go,CAFxX/go,josharian/go,codestation/go,josharian/go,akutz/go,CAFxX/go,Cofyc/go,josharian/go