nir/flrp: Fix typo on the flrp(±1, b, c) path

After Samuel reported the bisect, I was able to find the bug by
inspection.  Good thing for well-named varibles. :)

Unfortunately, this undoes almost all of the benefit of the original
patch.

Ice Lake
total instructions in shared programs: 17183159 -> 17218166 (0.20%)
instructions in affected programs: 1308722 -> 1343729 (2.67%)
helped: 98
HURT: 4746
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.47% max: 2.70% x̄: 0.60% x̃: 0.57%
HURT stats (abs)   min: 1 max: 691 x̄: 7.40 x̃: 8
HURT stats (rel)   min: 0.10% max: 700.00% x̄: 5.82% x̃: 2.83%
95% mean confidence interval for instructions value: 6.82 7.64
95% mean confidence interval for instructions %-change: 5.22% 6.15%
Instructions are HURT.

total cycles in shared programs: 360705959 -> 360853522 (0.04%)
cycles in affected programs: 10754380 -> 10901943 (1.37%)
helped: 1594
HURT: 3331
helped stats (abs) min: 1 max: 1896 x̄: 119.81 x̃: 60
helped stats (rel) min: <.01% max: 35.48% x̄: 5.06% x̃: 3.64%
HURT stats (abs)   min: 1 max: 10208 x̄: 101.63 x̃: 38
HURT stats (rel)   min: 0.01% max: 878.95% x̄: 9.01% x̃: 2.78%
95% mean confidence interval for cycles value: 21.11 38.81
95% mean confidence interval for cycles %-change: 3.76% 5.15%
Cycles are HURT.

total spills in shared programs: 12158 -> 12355 (1.62%)
spills in affected programs: 98 -> 295 (201.02%)
helped: 1
HURT: 2

total fills in shared programs: 25204 -> 25398 (0.77%)
fills in affected programs: 94 -> 288 (206.38%)
helped: 0
HURT: 3

LOST:   15
GAINED: 8

Iron Lake
total instructions in shared programs: 8121430 -> 8166733 (0.56%)
instructions in affected programs: 1148353 -> 1193656 (3.95%)
helped: 2
HURT: 4046
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 1.85% max: 1.92% x̄: 1.89% x̃: 1.89%
HURT stats (abs)   min: 1 max: 43 x̄: 11.20 x̃: 11
HURT stats (rel)   min: 0.20% max: 716.67% x̄: 7.40% x̃: 3.87%
95% mean confidence interval for instructions value: 11.02 11.37
95% mean confidence interval for instructions %-change: 6.84% 7.94%
Instructions are HURT.

total cycles in shared programs: 188376326 -> 188601568 (0.12%)
cycles in affected programs: 27416674 -> 27641916 (0.82%)
helped: 68
HURT: 3947
helped stats (abs) min: 2 max: 222 x̄: 13.88 x̃: 6
helped stats (rel) min: <.01% max: 1.28% x̄: 0.15% x̃: 0.01%
HURT stats (abs)   min: 2 max: 670 x̄: 57.31 x̃: 64
HURT stats (rel)   min: <.01% max: 1811.11% x̄: 4.11% x̃: 1.09%
95% mean confidence interval for cycles value: 55.01 57.20
95% mean confidence interval for cycles %-change: 2.88% 5.19%
Cycles are HURT.

LOST:   35
GAINED: 3

GM45
total instructions in shared programs: 4979794 -> 5003551 (0.48%)
instructions in affected programs: 635174 -> 658931 (3.74%)
helped: 1
HURT: 2142
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 1.85% max: 1.85% x̄: 1.85% x̃: 1.85%
HURT stats (abs)   min: 1 max: 43 x̄: 11.09 x̃: 11
HURT stats (rel)   min: 0.20% max: 716.67% x̄: 7.00% x̃: 3.53%
95% mean confidence interval for instructions value: 10.85 11.33
95% mean confidence interval for instructions %-change: 6.25% 7.74%
Instructions are HURT.

total cycles in shared programs: 128519586 -> 128654990 (0.11%)
cycles in affected programs: 17635304 -> 17770708 (0.77%)
helped: 46
HURT: 2088
helped stats (abs) min: 4 max: 220 x̄: 18.13 x̃: 6
helped stats (rel) min: <.01% max: 1.28% x̄: 0.15% x̃: 0.01%
HURT stats (abs)   min: 2 max: 670 x̄: 65.25 x̃: 66
HURT stats (rel)   min: <.01% max: 1464.29% x̄: 4.05% x̃: 0.99%
95% mean confidence interval for cycles value: 61.75 65.15
95% mean confidence interval for cycles %-change: 2.58% 5.34%
Cycles are HURT.

LOST:   38
GAINED: 38

Fixes: 5b908db604 ("nir/flrp: Lower flrp(±1, b, c) and flrp(a, ±1, c) differently")
Reported-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/compiler/nir/nir_lower_flrp.c

@@ -172,6 +172,8 @@ replace_with_fast(struct nir_builder *bld, struct u_vector *dead_flrp,
 
 /**
  * Replace flrp(a, b, c) with (b*c ± c) + a
+ *
+ * \note: This only works if a = ±1.
  */
 static void
 replace_with_expanded_ffma_and_add(struct nir_builder *bld,
@@ -182,7 +184,7 @@ replace_with_expanded_ffma_and_add(struct nir_builder *bld,
    nir_ssa_def *const b = nir_ssa_for_alu_src(bld, alu, 1);
    nir_ssa_def *const c = nir_ssa_for_alu_src(bld, alu, 2);
 
-   nir_ssa_def *const b_times_c = nir_fadd(bld, b, c);
+   nir_ssa_def *const b_times_c = nir_fmul(bld, b, c);
    nir_instr_as_alu(b_times_c->parent_instr)->exact = alu->exact;
 
    nir_ssa_def *inner_sum;