st/nir: Trim out unused VS input variables.

st/nir: Trim out unused VS input variables.

If we're going to skip setting up vertex input data in them, we should
probably not leave them as vertex inputs with a driver_location that
happens to alias to something else.

Fixes a regression in glsl-mat-attribute on vc4 when enabling GTN.

v2: Change commit message shortlog, lower the new globals away before
    handing off to the driver.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>