i965/fs: Improve live variables calculation performance.

i965/fs: Improve live variables calculation performance.

We can execute way fewer instructions by doing our boolean manipulation
on an "int" of bits at a time, while also reducing our working set size.

Reduces compile time of L4D2's slowest shader from 4s to 1.1s
(-72.4% +/- 0.2%, n=10)

v2: Remove redundant masking (noted by Ken)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>