nir: Silence a couple of warnings in release builds

nir: Silence a couple of warnings in release builds

[28/716] Compiling C object 'src/compiler/nir/068b2c8@@nir@sta/nir_gather_xfb_info.c.o'.
../src/compiler/nir/nir_gather_xfb_info.c: In function ‘nir_gather_xfb_info’:
../src/compiler/nir/nir_gather_xfb_info.c:171:13: warning: variable ‘max_offset’ set but not used [-Wunused-but-set-variable]
    unsigned max_offset[NIR_MAX_XFB_BUFFERS] = {0};
             ^~~~~~~~~~
[36/716] Compiling C object 'src/compiler/nir/068b2c8@@nir@sta/nir_instr_set.c.o'.
../src/compiler/nir/nir_instr_set.c:502:1: warning: ‘instr_each_src_and_dest_is_ssa’ defined but not used [-Wunused-function]
 instr_each_src_and_dest_is_ssa(nir_instr *instr)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>