nir/lower_viewport: Check variable mode first

The location is unused for shader_temp and function_temp variables, and
due to the way we nir_lower_io_to_temproraries demotes shader_out
variables to shader_temp variables, it happened to equal
VARYING_SLOT_POS for the gl_Position temporary, which made this pass
fail with the offline compiler due to this coming before vars_to_ssa.

Reviewed-by: Qiang Yu <yuq825@gmail.com>
1 file changed