intel/fs,vec4: Fix a compiler warning

intel/fs,vec4: Fix a compiler warning

../src/intel/compiler/brw_fs_nir.cpp:3534:46: warning: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Wsign-compare]
       assert(nir_intrinsic_write_mask(instr) ==
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
              (1 << instr->num_components) - 1);
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This was caused by 6339aba775ecdc which added these completely valid
checks.  However clang likes to complain about signedness mismatches.

Fixes: 6339aba775ecdc "intel/compiler: Lower SSBO and shared..."
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>