i965/fs: Compact the virtual GRF arrays.

During code generation, we create tons of temporary variables, many of
which get immediately killed and are never used.  Later optimization and
analysis passes, such as compute_live_intervals, loop over all the
virtual GRFs.  By compacting them, we can save a lot of overhead.

Reduces compilation time in L4D2's largest fragment shader from 10.2
seconds to 5.2 seconds (50%).  Drops compute_live_variables() from
10-12% of another game's startup time to 8%.

Reviewed-by: Eric Anholt <[email protected]>
Signed-off-by: Kenneth Graunke <[email protected]>

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