This makes the role of promote() in the system clearer and helps to make
the code more understandable, since more of the checks are in the same
logical place.

BUG=angleproject:952
TEST=angle_unittests, WebGL conformance tests

Change-Id: Idb2de927d872e46210d71cf6de06a6f8c1fc5da1
Reviewed-on: https://chromium-review.googlesource.com/260803Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>