When reading vertex attributes, GLES requires the y and z components to
be filled with 0 if missing, and the alpha channel to be filled with 1.0
for float types and 1 for integer types.

At the same time, this change reduces the number of variations in this
shader.

Bug: angleproject:3192
Change-Id: I1bdf491952c296962fa9160f2d29d415eb3d570d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1936708Reviewed-by: Jaedon Lee <jaedon1.lee@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>