math: faster Gamma
math: faster Gamma

Having the compiler count the number of array elements speeds up Gamma from 63.7 to 56.6 ns/op.

R=rsc, golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5362043

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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