Always size input layouts to gl::MAX_VERTEX_ATTRIBS, and use '1' bits
to signal where we have a GPU conversion. The simplification allows us
to more cleanly match the vertex executable signatures and makes our
default VertexExecutable hit much more often.

BUG=510151
TEST=angle_end2end_tests,Canary WebGL, manual testing with Chromium

Change-Id: I5009323c4e7e208e7a2595be46658c344517a4ff
Reviewed-on: https://chromium-review.googlesource.com/290740Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>