big: eliminate redundant array lookups
big: eliminate redundant array lookups

This gives about a 6% performance improvement to pidigits.
Thanks to Russ for the suggestion.

R=rsc, gri
CC=golang-dev
http://codereview.appspot.com/957041

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go