Any time we would query ProgramD3D for a particular vertex or pixel
shader, we would iterate over all the cached shaders of the program
looking for the matching input or output layout signature. This change
makes it so we only compute the index of the matching shader once,
and subsequent calls will re-use the shader index. This should speed
up the draw call benchmarks.

Also include a fix to the Serial class that initializes a Serial value
to an invalid sentinel value. This ensures that comparing any other
Serial (including another invalid serial) to the invalid serial will
return not-equals.

BUG=angleproject:1155

Change-Id: I7d913bf08d0bedf6155eae0661b2a5fa94565cc9
Reviewed-on: https://chromium-review.googlesource.com/648730
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>