Make NVIDIA constant register zero skip specific to VS It was found that the driver issue only affects vertex shaders, so the workaround can be simplified to affect only vertex shaders. BUG=angleproject:2294 TEST=WebGL conformance tests on passthrough command buffer, angle_end2end_tests Change-Id: Ie7fb4e75a3999e1de3d5c20d3de21e7ebb08f148 Reviewed-on: https://chromium-review.googlesource.com/889099 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>