D3D10+ has stricter rules on linking shader inputs with outputs than
the previous comments in the code indicated. Add comments and checks
related to this and structure the code in a way that makes it a bit
less prone to errors.

This page is intended to document the rules though it is somewhat
vague:
https://docs.microsoft.com/en-us/windows/desktop/direct3dhlsl/dx-graphics-hlsl-signatures

BUG=angleproject:2767
TEST=angle_end2end_tests

Change-Id: I236ec4cd5cbf3889fd2c97947ee81a6c5ae6a787
Reviewed-on: https://chromium-review.googlesource.com/1169818Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>