llvmpipe: Generalize the x8z24 fast path to all depth formats.

llvmpipe: Generalize the x8z24 fast path to all depth formats.

Together with the previous commit, this generalize the benefits of
d2cf757f44f4ee5554243f3279483a25886d9927 to all depth formats, in
particular:
- simpler float -> 24unorm conversion
- avoid unsigned comparisons (not directly supported on SSE) by aligning
to the least significant bit
- avoid unecessary/repeated mask ANDing

Verified with trivial/tri-z that the exact same assembly is produced for
X8Z24.