This avoids one more function call + register pushing/popping. Slight
improvement in the command_buffer_perftests.

Bug: angleproject:2877
Change-Id: Ia50a330e62f0bd32266c9f0999810243cbaa41e0
Reviewed-on: https://chromium-review.googlesource.com/c/1293630Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>