i965: Don't make instructions with a null dest a barrier to scheduling.

i965: Don't make instructions with a null dest a barrier to scheduling.

Now that we properly track accumulator dependencies, the scheduler is
able to schedule instructions between the mach and mov in the common
the integer multiplication pattern:

   mul  acc0, x, y
   mach null, x, y
   mov  dest, acc0

Since a null destination implies no dependency on the destination, we
can also safely schedule instructions (that don't write the accumulator)
between the mul and mach.

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