We would generate multiple vertex binaries that result in the same
HLSL code, eg for vec2 and vec3 vertex attributes. Eliminate
duplicates by using the converted attribute type for testing
for matching binaries.

BUG=angle:599

Change-Id: I061588164577ff9fa69ebb7d8a3f2bf6bb6fe013
Reviewed-on: https://chromium-review.googlesource.com/197830Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>