nir: Add support for 2src_commutative ops that have 3 sources

v2: Instead of handling 3 sources as a special case, generalize with
loops to N sources.  Suggested by Jason.

v3: Further generalize by only checking that number of sources is >= 2.
Suggested by Jason.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_search.c

@@ -408,7 +408,11 @@ match_expression(const nir_search_expression *expr, nir_alu_instr *instr,
 
    bool matched = true;
    for (unsigned i = 0; i < nir_op_infos[instr->op].num_inputs; i++) {
-      if (!match_value(expr->srcs[i], instr, i ^ comm_op_flip,
+      /* 2src_commutative instructions that have 3 sources are only commutative
+       * in the first two sources.  Source 2 is always source 2.
+       */
+      if (!match_value(expr->srcs[i], instr,
+                       i < 2 ? i ^ comm_op_flip : i,
                        num_components, swizzle, state)) {
          matched = false;
          break;