D3D11 Feature Level 9_3 supports instancing, but slot 0 in the input
layout must not be instanced. D3D9 has a similar restriction, where
stream 0 must not be instanced. This restriction can be worked around
by remapping any non-instanced slot to slot 0. This works because
HLSL uses shader semantics to match the vertex inputs to the elements
in the input layout, rather than the slots.

BUG=angle:858

Change-Id: I67b2be9095afc206a4b9f107ed61356820551afe
Reviewed-on: https://chromium-review.googlesource.com/237270Tested-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>