intel/compiler: More DCE after lowering

intel/compiler: More DCE after lowering

Some of the lowering passes, nir_lower_locals_to_regs for example, can
cause some previously live code to be dead.  This pass in particular
leaves a bunch of nir_instr_type_deref instructions floating around.
This causes shader-db runs on Gen5 through Haswell to spew tons of
messages like:

    VS instruction not yet implemented by NIR->vec4

UnrealEngine4/EffectsCaveDemo/239.shader_test is one shader that
generates these messages.  Cleaning up the dead code fixes that.

To verify, I did a shader-db before and after.  Even though all the
messages are gone, the results make my brain hurt. :(

Haswell
total cycles in shared programs: 411890163 -> 411891145 (<.01%)
cycles in affected programs: 57016 -> 57998 (1.72%)
helped: 3
HURT: 11
helped stats (abs) min: 2 max: 154 x̄: 96.67 x̃: 134
helped stats (rel) min: 0.08% max: 2.23% x̄: 1.42% x̃: 1.96%
HURT stats (abs)   min: 18 max: 686 x̄: 115.64 x̃: 20
HURT stats (rel)   min: 0.81% max: 7.12% x̄: 1.87% x̃: 0.93%
95% mean confidence interval for cycles value: -51.39 191.67
95% mean confidence interval for cycles %-change: -0.14% 2.46%
Inconclusive result (value mean confidence interval includes 0).

Ivy Bridge
total cycles in shared programs: 259114802 -> 259115032 (<.01%)
cycles in affected programs: 24034 -> 24264 (0.96%)
helped: 1
HURT: 9
helped stats (abs) min: 2 max: 2 x̄: 2.00 x̃: 2
helped stats (rel) min: 0.08% max: 0.08% x̄: 0.08% x̃: 0.08%
HURT stats (abs)   min: 18 max: 48 x̄: 25.78 x̃: 20
HURT stats (rel)   min: 0.80% max: 1.94% x̄: 1.08% x̃: 0.80%
95% mean confidence interval for cycles value: 12.42 33.58
95% mean confidence interval for cycles %-change: 0.54% 1.38%
Cycles are HURT.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Fixes: 5a02ffb733e nir: Rework lower_locals_to_regs to use deref instructions
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>