For the case of drawing with un-normalized integer vertex attributes,
we need to do some dynamic conversion in the VS. However, each
attribute can either be signed or unsigned, and our shader signature
code would treat both as a match, giving rise to a warning in the
D3D11 Debug runtime. It's unclear if this would give incorrect
results, but it certainly should not produce a warning.

BUG=angleproject:1329

Change-Id: I302d11b44e8a0ef981e89c181aefac5451a899b7
Reviewed-on: https://chromium-review.googlesource.com/329998
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>