nir: Make nir_lower_clip_vs optionally work with variables.

nir: Make nir_lower_clip_vs optionally work with variables.

The way nir_lower_clip_vs() works with store_output intrinsics makes a
ton of assumptions about the driver_location field.

In i965 and iris, I'd rather do this lowering early and work with
variables.  v3d may want to switch to that as well, and ir3 could too,
but I'm not sure exactly what would need updating.  For now, handle
both methods.

Reviewed-by: Eric Anholt <eric@anholt.net>