This was causing a heavy performance regression in the Vulkan perf
test. Local testing shows it recovers most of the performance gap
with this fix.

Bug: angleproject:2264
Bug: chromium:799372
Change-Id: Ia2bbf526b305e1b2e901d904574dbe5618e6e5ef
Reviewed-on: https://chromium-review.googlesource.com/852365Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>