The order of computation was switched unintentionally
in https://go-review.googlesource.com/21910.

Revert the order to first compute g then b.

Change-Id: I8cedb5e45fbad2679246839f609bcac4f9052403
Reviewed-on: https://go-review.googlesource.com/22016Reviewed-by: Nigel Tao <nigeltao@golang.org>