i965: Use nir_lower_load_const_to_scalar().

I don't know why, but we never hooked up this pass Eric wrote.
Otherwise, you can end up with stupid scalarized code such as:

   vec4 ssa_7 = load_const (0.0, 0.0, 0.0, 0.0)
   vec4 ssa_8 = ...
   vec1 ssa_9 = feq ssa_8, ssa_7
   vec1 ssa_10 = feq ssa_8.y, ssa_7.y
   vec1 ssa_11 = feq ssa_8, ssa_7.z
   vec1 ssa_12 = feq ssa_8.y, ssa_7.w

ssa_8.xyxy == <0, 0, 0, 0> should only take two feq instructions.

shader-db on Skylake:

total instructions in shared programs: 9121153 -> 9120749 (-0.00%)
instructions in affected programs: 32421 -> 32017 (-1.25%)
helped: 277
HURT: 69

total cycles in shared programs: 69003364 -> 69000912 (-0.00%)
cycles in affected programs: 899186 -> 896734 (-0.27%)
helped: 313
HURT: 403

This also prevents regressions when disabling channel expressions.

v2: Don't call opt_cse afterwards (requested by Matt).  It should
    happen in the optimization loop below anyway.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Eduardo Lima Mitev <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

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