nir: Use nir_src_bit_size instead of alu1->dest.dest.ssa.bit_size

This is important because, for example nir_op_fne has
dest.dest.ssa.bit_size == 1, but the source operands can be 16-, 32-, or
64-bits.  Fixing this helps partial redundancy elimination for compares
in a few more shaders.

v2: Add unit tests for nir_opt_comparison_pre that are fixed by this
commit.

All Intel platforms had similar results.
total instructions in shared programs: 17179408 -> 17179081 (<.01%)
instructions in affected programs: 43958 -> 43631 (-0.74%)
helped: 118
HURT: 2
helped stats (abs) min: 1 max: 5 x̄: 2.87 x̃: 2
helped stats (rel) min: 0.06% max: 4.12% x̄: 1.19% x̃: 0.81%
HURT stats (abs)   min: 6 max: 6 x̄: 6.00 x̃: 6
HURT stats (rel)   min: 5.83% max: 6.06% x̄: 5.94% x̃: 5.94%
95% mean confidence interval for instructions value: -3.08 -2.37
95% mean confidence interval for instructions %-change: -1.30% -0.85%
Instructions are helped.

total cycles in shared programs: 360959066 -> 360942386 (<.01%)
cycles in affected programs: 774274 -> 757594 (-2.15%)
helped: 111
HURT: 4
helped stats (abs) min: 1 max: 1591 x̄: 169.49 x̃: 36
helped stats (rel) min: <.01% max: 24.43% x̄: 8.86% x̃: 2.24%
HURT stats (abs)   min: 1 max: 2068 x̄: 533.25 x̃: 32
HURT stats (rel)   min: 0.02% max: 5.10% x̄: 3.06% x̃: 3.56%
95% mean confidence interval for cycles value: -200.61 -89.47
95% mean confidence interval for cycles %-change: -10.32% -6.58%
Cycles are helped.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net> [v1]
Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Fixes: be1cc355 ("nir: Add nir_const_value_negative_equal")
(cherry picked from commit 0ac5ff9e)