nir: Reorder ffma and fsub combining

nir: Reorder ffma and fsub combining

It's relatively common to do something like "a * b - c", which on most
GPUs can be implemented in a single instruction. Before
opt_algebraic_late this will be something like
"fadd(fmul(a, b), fneg(c))", and we want to turn it info
"ffma(a, b, fneg(c))". But because the fsub pattern was first we
instead turned it into "fsub(fmul(a, b), c)". Fix this by reordering
them.

Selected shader-db results on freedreno:

total instructions in shared programs: 1561330 -> 1551619 (-0.62%)
instructions in affected programs: 780272 -> 770561 (-1.24%)
helped: 1941
HURT: 491
helped stats (abs) min: 1 max: 147 x̄: 7.98 x̃: 4
helped stats (rel) min: 0.07% max: 30.77% x̄: 4.36% x̃: 3.17%
HURT stats (abs)   min: 1 max: 307 x̄: 11.76 x̃: 5
HURT stats (rel)   min: 0.09% max: 18.71% x̄: 2.26% x̃: 1.38%
95% mean confidence interval for instructions value: -4.57 -3.41
95% mean confidence interval for instructions %-change: -3.21% -2.84%
Instructions are helped.

total nops in shared programs: 358926 -> 356263 (-0.74%)
nops in affected programs: 167116 -> 164453 (-1.59%)
helped: 1395
HURT: 859
helped stats (abs) min: 1 max: 108 x̄: 6.80 x̃: 3
helped stats (rel) min: 0.17% max: 100.00% x̄: 19.15% x̃: 10.57%
HURT stats (abs)   min: 1 max: 307 x̄: 7.95 x̃: 3
HURT stats (rel)   min: 0.00% max: 381.82% x̄: 20.04% x̃: 10.00%
95% mean confidence interval for nops value: -1.77 -0.59
95% mean confidence interval for nops %-change: -5.55% -2.87%
Nops are helped.

total non-nops in shared programs: 1202404 -> 1195356 (-0.59%)
non-nops in affected programs: 496682 -> 489634 (-1.42%)
helped: 1951
HURT: 265
helped stats (abs) min: 1 max: 39 x̄: 4.02 x̃: 3
helped stats (rel) min: 0.07% max: 15.38% x̄: 2.97% x̃: 1.96%
HURT stats (abs)   min: 1 max: 22 x̄: 2.97 x̃: 2
HURT stats (rel)   min: 0.05% max: 10.00% x̄: 1.14% x̃: 0.75%
95% mean confidence interval for non-nops value: -3.38 -2.99
95% mean confidence interval for non-nops %-change: -2.60% -2.36%
Non-nops are helped.

total systall in shared programs: 288317 -> 292975 (1.62%)
systall in affected programs: 87876 -> 92534 (5.30%)
helped: 388
HURT: 431
helped stats (abs) min: 1 max: 214 x̄: 14.39 x̃: 8
helped stats (rel) min: 0.25% max: 100.00% x̄: 22.12% x̃: 11.96%
HURT stats (abs)   min: 1 max: 232 x̄: 23.77 x̃: 7
HURT stats (rel)   min: 0.00% max: 1300.00% x̄: 51.71% x̃: 17.30%
95% mean confidence interval for systall value: 3.07 8.30
95% mean confidence interval for systall %-change: 9.49% 23.97%
Systall are HURT.

(The systall hurt is probably just due to having having fewer
instructions to hide latency with.)

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14554>