intel/fs: Skip registers faster when setting spill costs

intel/fs: Skip registers faster when setting spill costs

This might be slightly faster since we're doing one read rather than
two before we decide to skip.  The more important reason, however, is
because no_spill prevents us from re-spilling spill registers.  In the
new world in which we don't re-calculate liveness every spill, we may
not have valid liveness for spill registers so we shouldn't even look
their live ranges up.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110825
Fixes: e99081e76d4 "intel/fs/ra: Spill without destroying the..."
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Tested-by: Tapani Pälli <tapani.palli@intel.com>