i965/fs: Remove start/end aliases in compute_live_intervals().

i965/fs: Remove start/end aliases in compute_live_intervals().

In compute_live_intervals(), start and end are shorter names for
the virtual_grf_start and virtual_grf_end class members.

Now that the fs_live_intervals class has arrays named start and end
which are indexed by var, rather than VGRF, reusing the name is
confusing.  Plus, most of the code has been factored out, so using the
long names isn't as inconvenient.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>