i965: use nir_lower_indirect_derefs() for GLSL

i965: use nir_lower_indirect_derefs() for GLSL

This moves the nir_lower_indirect_derefs() call into
brw_preprocess_nir() so thats is called by both OpenGL and Vulkan
and removes that call to the old GLSL IR pass
lower_variable_index_to_cond_assign()

We want to do this pass in nir to be able to move loop unrolling
to nir.

There is a increase of 1-3 instructions in a small number of shaders,
and 2 Kerbal Space program shaders that increase by 32 instructions.
The changes seem to be caused be the difference in the GLSL IR vs
NIR variable index lowering passes. The GLSL IR pass creates a
simple if ladder for arrays of size 4 or less, while the NIR pass
implements a binary search for all arrays regardless of size.

Shader-db results BDW:

total instructions in shared programs: 13021176 -> 13021819 (0.00%)
instructions in affected programs: 57693 -> 58336 (1.11%)
helped: 20
HURT: 190

total cycles in shared programs: 299805580 -> 299750826 (-0.02%)
cycles in affected programs: 2290024 -> 2235270 (-2.39%)
helped: 337
HURT: 442

total fills in shared programs: 19984 -> 19984 (0.00%)
fills in affected programs: 0 -> 0
helped: 0
HURT: 0

LOST:   4
GAINED: 0

V2: remove the do_copy_propagation() call from the i965 GLSL IR
linking code. This call was added in f7741c52111 but since we are
moving the variable index lowering to NIR we no longer need it and
can just rely on the nir copy propagation pass.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>