anv: Move nir_lower_wpos_center after dead variable elimination.

anv: Move nir_lower_wpos_center after dead variable elimination.

When multiple shader stages exist in the same SPIR-V module, we compile
all entry points and their inputs/outputs, then dead code eliminate the
ones not related to the specific entry point later.

nir_lower_wpos_center was being run prior to eliminating those random
other variables, which made it trip up, thinking it found gl_FragCoord
when it actually found something else like gl_PerVertex[3].

Fixes dEQP-VK.spirv_assembly.instruction.graphics.module.same_module.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>