Constructor arguments don't need to be rounded if the constructor
result will be rounded to the same precision.

This will make precision emulated shaders slightly faster to execute
in some cases.

BUG=angleproject:874
TEST=angle_unittests

Change-Id: I4220cd2289c97dcf5b7a25a4cbdd18487947c2d2
Reviewed-on: https://chromium-review.googlesource.com/500288Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>