intel/compiler: don't use a keyword struct for a class fs_reg

warning: struct 'fs_reg' was previously declared as a class
Fixes: e64be391 ("intel/compiler: generalize the combine constants pass")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
(cherry picked from commit fa2fc68d)