intel/fs: Skip registers faster when setting spill costs

Faith Ekstrand requested to merge gfxstrand/mesa:review/bug-110825 into master

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.

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