r600/sfn: Fix a few warnings in release builds

r600/sfn: Fix a few warnings in release builds

[1083/1142] Compiling C++ object src/gallium/drivers/r600/libr600.a.p/sfn_sfn_nir_lower_fs_out_to_vector.cpp.o
../src/gallium/drivers/r600/sfn/sfn_nir_lower_fs_out_to_vector.cpp: In member function ‘nir_ssa_def* r600::NirLowerFSOutToVector::create_combined_vector(nir_builder*, nir_ssa_def**, int, int)’:
../src/gallium/drivers/r600/sfn/sfn_nir_lower_fs_out_to_vector.cpp:442:48: warning: ‘op’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 nir_alu_instr * instr = nir_alu_instr_create(b->shader, op);
      |                            ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~

[1092/1142] Compiling C++ object src/gallium/drivers/r600/libr600.a.p/sfn_sfn_nir.cpp.o
../src/gallium/drivers/r600/sfn/sfn_nir.cpp: In function ‘int r600_shader_from_nir(r600_context*, r600_pipe_shader*, r600_shader_key*)’:
../src/gallium/drivers/r600/sfn/sfn_nir.cpp:859:24: warning: unused variable ‘func’ [-Wunused-variable]
  859 |    const nir_function *func = reinterpret_cast<const nir_function *>(exec_list_get_head_const(&sel->nir->functions));
      |                        ^~~~

[1118/1142] Compiling C++ object src/gallium/drivers/r600/libr600.a.p/sfn_sfn_shader_base.cpp.o
../src/gallium/drivers/r600/sfn/sfn_shader_base.cpp: In member function ‘void r600::ShaderFromNirProcessor::add_array_deref(nir_deref_instr*)’:
../src/gallium/drivers/r600/sfn/sfn_shader_base.cpp:300:18: warning: unused variable ‘var’ [-Wunused-variable]
  300 |    nir_variable *var = nir_deref_instr_get_variable(instr);

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7820>