Instead of multiple ifs that test for various combination of shader
stages and calling AssignVaryingLocations for the appropriate pairs,
that function loops over all stages and assigns a location to the
varying in any stage it is active.

This change will help keep this code simpler with the introduction of
tessellation shaders.

Bug: angleproject:3571
Change-Id: Iaf3ee67990bd219080314237b314bd2770a43085
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1961096Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>