intel/compiler: Repeat nir_opt_algebraic_late

A tiny bit of help seems to come from nir_copy_prop.  Future patches
will benefit from this change.

Doing more copy propagation on the vec4 backend led to a disaster in
hurt cycles.

v2: Fix typo in comment.  Noticed by Matt.

All Gen8+ platforms had similar results. (Ice Lake shown)
total instructions in shared programs: 17224634 -> 17224623 (<.01%)
instructions in affected programs: 4586 -> 4575 (-0.24%)
helped: 11
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.19% max: 0.53% x̄: 0.27% x̃: 0.23%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -0.36% -0.19%
Instructions are helped.

total cycles in shared programs: 360828542 -> 360828714 (<.01%)
cycles in affected programs: 151159 -> 151331 (0.11%)
helped: 49
HURT: 28
helped stats (abs) min: 1 max: 254 x̄: 26.41 x̃: 6
helped stats (rel) min: 0.06% max: 12.02% x̄: 1.34% x̃: 0.42%
HURT stats (abs)   min: 1 max: 196 x̄: 52.36 x̃: 15
HURT stats (rel)   min: 0.05% max: 10.74% x̄: 2.55% x̃: 0.88%
95% mean confidence interval for cycles value: -13.48 17.95
95% mean confidence interval for cycles %-change: -0.69% 0.84%
Inconclusive result (value mean confidence interval includes 0).

Haswell, Ivy Bridge, and Sandy Bridge had similar results. (Haswell shown)
total instructions in shared programs: 13529544 -> 13529542 (<.01%)
instructions in affected programs: 358 -> 356 (-0.56%)
helped: 2
HURT: 0

total cycles in shared programs: 357290311 -> 357289678 (<.01%)
cycles in affected programs: 178324 -> 177691 (-0.35%)
helped: 48
HURT: 40
helped stats (abs) min: 1 max: 201 x̄: 31.52 x̃: 13
helped stats (rel) min: 0.06% max: 10.92% x̄: 1.71% x̃: 0.66%
HURT stats (abs)   min: 1 max: 224 x̄: 22.00 x̃: 6
HURT stats (rel)   min: 0.05% max: 15.84% x̄: 1.29% x̃: 0.31%
95% mean confidence interval for cycles value: -18.28 3.89
95% mean confidence interval for cycles %-change: -1.01% 0.32%
Inconclusive result (value mean confidence interval includes 0).

Iron Lake and GM45 had similar results. (Iron Lake shown)
total instructions in shared programs: 8159110 -> 8158980 (<.01%)
instructions in affected programs: 22719 -> 22589 (-0.57%)
helped: 65
HURT: 0
helped stats (abs) min: 1 max: 3 x̄: 2.00 x̃: 2
helped stats (rel) min: 0.07% max: 1.05% x̄: 0.73% x̃: 0.74%
95% mean confidence interval for instructions value: -2.06 -1.94
95% mean confidence interval for instructions %-change: -0.78% -0.68%
Instructions are helped.

total cycles in shared programs: 188609448 -> 188609214 (<.01%)
cycles in affected programs: 1875852 -> 1875618 (-0.01%)
helped: 109
HURT: 104
helped stats (abs) min: 2 max: 46 x̄: 5.30 x̃: 4
helped stats (rel) min: 0.02% max: 0.90% x̄: 0.09% x̃: 0.07%
HURT stats (abs)   min: 2 max: 20 x̄: 3.31 x̃: 2
HURT stats (rel)   min: 0.01% max: 0.26% x̄: 0.04% x̃: 0.02%
95% mean confidence interval for cycles value: -1.95 -0.25
95% mean confidence interval for cycles %-change: -0.04% -0.01%
Cycles are helped.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/intel/compiler/brw_nir.c

@@ -912,7 +912,22 @@ brw_postprocess_nir(nir_shader *nir, const struct brw_compiler *compiler,
           compiler->devinfo->gen >= 6);
    }
 
-   OPT(nir_opt_algebraic_late);
+   do {
+      progress = false;
+      if (OPT(nir_opt_algebraic_late)) {
+         /* At this late stage, anything that makes more constants will wreak
+          * havok on the vec4 backend.  The handling of constants in the vec4
+          * backend is not good.
+          */
+         if (is_scalar) {
+            OPT(nir_opt_constant_folding);
+            OPT(nir_copy_prop);
+         }
+         OPT(nir_opt_dce);
+         OPT(nir_opt_cse);
+      }
+   } while (progress);
+
 
    OPT(brw_nir_lower_conversions);