Instead of generating a struct each time we can use a constexpr
constructor to use a single representation.

Bug: angleproject:3024
Change-Id: I14dec65a4f6ac9ab2f7e7af444862e4ceab88d8c
Reviewed-on: https://chromium-review.googlesource.com/c/1392395Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>