Uses inlining and more efficient computation re-use to streamline this
function.

Bug: angleproject:2575
Change-Id: Ib13e32811f56ec9a010ed66f298d4235e5c6807d
Reviewed-on: https://chromium-review.googlesource.com/1067120Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>