intel/compiler: fix -Wsign-compare warning

intel/compiler: fix -Wsign-compare warning

Explicitly convert to signed integer. Conversion is valid since is the
same (implicitly) used to initialize the loop. Avoids the warning:

../../src/intel/compiler/brw_fs.cpp: In member function ‘bool fs_visitor::lower_simd_width()’:
../../src/intel/compiler/brw_fs.cpp:5761:45: warning: comparison of integer expressions of different signedness: ‘int’ and ‘unsigned int’ [-Wsign-compare]
             split_inst.eot = inst->eot && i == n - 1;
                                           ~~^~~~~~~~

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>