brw_nir: Cast bitshift to unsigned

brw_nir: Cast bitshift to unsigned

../src/intel/compiler/brw_nir.c:979:40: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'
    #0 0x7f78f590d10b in brw_nir_apply_sampler_key ../src/intel/compiler/brw_nir.c:979
    #1 0x7f78f590e07b in brw_nir_apply_key ../src/intel/compiler/brw_nir.c:1057
    #2 0x7f78f5754b45 in brw_compile_fs ../src/intel/compiler/brw_fs.cpp:8347
    #3 0x7f78f255c8e4 in brw_codegen_wm_prog ../src/mesa/drivers/dri/i965/brw_wm.c:123
    #4 0x7f78f2565571 in brw_fs_precompile ../src/mesa/drivers/dri/i965/brw_wm.c:608
    #5 0x7f78f24edd2c in brw_shader_precompile ../src/mesa/drivers/dri/i965/brw_link.cpp:56
    #6 0x7f78f24f3af8 in brw_link_shader ../src/mesa/drivers/dri/i965/brw_link.cpp:381
    #7 0x7f78f39a302a in _mesa_glsl_link_shader ../src/mesa/program/ir_to_mesa.cpp:3119
    #8 0x7f78f3a43826 in create_new_program ../src/mesa/main/ff_fragment_shader.cpp:1133
    #9 0x7f78f3a43d00 in _mesa_get_fixed_func_fragment_program ../src/mesa/main/ff_fragment_shader.cpp:1163
    #10 0x7f78f325ddcd in update_program ../src/mesa/main/state.c:134
    #11 0x7f78f325fe64 in _mesa_update_state_locked ../src/mesa/main/state.c:360
    #12 0x7f78f32600f1 in _mesa_update_state ../src/mesa/main/state.c:394
    #13 0x7f78f2b3e587 in clear ../src/mesa/main/clear.c:169
    #14 0x7f78f2b3e587 in _mesa_Clear ../src/mesa/main/clear.c:242

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3825>