nir: Silence unused parameter warnings

nir: Silence unused parameter warnings

These cases had the parameter removed:

nir/nir_lower_vec_to_movs.c: In function ‘try_coalesce’:
nir/nir_lower_vec_to_movs.c:124:66: warning: unused parameter ‘shader’ [-Wunused-parameter]
 try_coalesce(nir_alu_instr *vec, unsigned start_idx, nir_shader *shader)
                                                                  ^
nir/nir_lower_io.c: In function ‘load_op’:
nir/nir_lower_io.c:147:32: warning: unused parameter ‘state’ [-Wunused-parameter]
 load_op(struct lower_io_state *state,
                                ^

These cases had the parameter (void) silenced because the parameter was
necessary for an interface:

nir/glsl_to_nir.cpp:1900:32: warning: unused parameter 'ir' [-Wunused-parameter]
 nir_visitor::visit(ir_barrier *ir)
                                ^
nir/nir.c: In function ‘remove_use_cb’:
nir/nir.c:802:35: warning: unused parameter ‘state’ [-Wunused-parameter]
 remove_use_cb(nir_src *src, void *state)
                                   ^
nir/nir.c: In function ‘remove_def_cb’:
nir/nir.c:811:37: warning: unused parameter ‘state’ [-Wunused-parameter]
 remove_def_cb(nir_dest *dest, void *state)
                                     ^

Number of total warnings in my build reduced from 2543 to 2538
(reduction of 5).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>