nir/loop_analyze: do not count fmul towards the limit when only used by fadd

As always with loop unrolling, don't look too closely at stats, but they
confirm more loops are now unrolled.

Foz-DB Navi10:
Totals from 66 (0.09% of 72319) affected shaders:
MaxWaves: 1464 -> 1424 (-2.73%); split: +0.82%, -3.55%
Instrs: 101778 -> 173128 (+70.10%)
CodeSize: 544148 -> 905392 (+66.39%)
VGPRs: 3652 -> 3788 (+3.72%); split: -0.77%, +4.49%
SpillSGPRs: 105 -> 75 (-28.57%)
Latency: 1197088 -> 1033471 (-13.67%); split: -17.08%, +3.41%
InvThroughput: 315257 -> 293245 (-6.98%); split: -13.29%, +6.31%
VClause: 1663 -> 3057 (+83.82%); split: -0.12%, +83.94%
SClause: 2797 -> 4496 (+60.74%); split: -0.21%, +60.96%
Copies: 6472 -> 11219 (+73.35%); split: -0.08%, +73.42%
Branches: 2695 -> 4697 (+74.29%); split: -0.56%, +74.84%
PreSGPRs: 3418 -> 3619 (+5.88%); split: -0.79%, +6.67%
PreVGPRs: 3305 -> 3423 (+3.57%); split: -1.06%, +4.63%
VALU: 73061 -> 124934 (+71.00%)
SALU: 11775 -> 20803 (+76.67%); split: -0.99%, +77.66%
VMEM: 2729 -> 4627 (+69.55%)
SMEM: 3796 -> 5869 (+54.61%); split: -0.18%, +54.79%

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41348>

src/compiler/nir/nir_loop_analyze.c

@@ -26,6 +26,7 @@
 #include "util/ralloc.h"
 #include "nir.h"
 #include "nir_constant_expressions.h"
+#include "nir_search_helpers.h"
 
 typedef struct {
    /* The loop we store information for */
@@ -97,6 +98,10 @@ instr_cost(loop_info_state *state, nir_instr *instr,
    } else if (nir_op_is_vec_or_mov(alu->op)) {
       /* movs and vecs are likely free. */
       return 0;
+   } else if ((alu->op == nir_op_fmul || alu->op == nir_op_fmulz) &&
+              is_only_used_by_fadd(alu)) {
+      /* If we can fuse fma/mad, do not count the mul. */
+      return 0;
    }
 
    if (alu->op == nir_op_flrp) {