i965/compiler: Use the new nir_opt_copy_prop_vars pass

We run this after nir_lower_vars_to_ssa so that as many load/store_var
intrinsics as possible before copy_prop_vars executes.  This is because the
pass isn't particularly efficient (it does a lot of linear walks of a
linked list) so we'd like as much of the work as possible to be done before
copy_prop_vars runs.

Shader DB results on Sky Lake:

   total instructions in shared programs: 12020290 -> 12013627 (-0.06%)
   instructions in affected programs: 26033 -> 19370 (-25.59%)
   helped: 16
   HURT: 13

   total cycles in shared programs: 137772848 -> 137549012 (-0.16%)
   cycles in affected programs: 6955660 -> 6731824 (-3.22%)
   helped: 217
   HURT: 237

   total loops in shared programs: 3208 -> 3208 (0.00%)
   loops in affected programs: 0 -> 0
   helped: 0
   HURT: 0

   total spills in shared programs: 4112 -> 4057 (-1.34%)
   spills in affected programs: 483 -> 428 (-11.39%)
   helped: 2
   HURT: 0

   total fills in shared programs: 5519 -> 5102 (-7.56%)
   fills in affected programs: 993 -> 576 (-41.99%)
   helped: 2
   HURT: 0

   LOST:   0
   GAINED: 0

Broadwell had similar results.  On older hardware, the impact isn't as
large because they don't advertise GL 4.5.  Of the hurt programs, all but
one are hurt by a single instruction and the one is hurt by 3 instructions.
All of the helped programs, on the other hand, are helped by at least 3
instructions and one kerbal space program shader is helped by 44.59%.

The real star of the show, however, is the Gl43CSDof synmark2 benchmark
which has two shaders which are cut by 28% and 40% and the over-all runtime
performance of the benchmark on my Sky Lake laptop is improved by around
25-30% (it's a bit hard to be exact due to thermal throttling).

Reviewed-by: Timothy Arceri <[email protected]>

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