Fix rnd bit generation in rmat_rectangular_kernel (#2524)

For certain architectures, the compiler always generates zero destination bit in the following loop https://github.com/rapidsai/raft/blob/ee45ce786686b54d1972408b927d7fcd8ce0cf20/cpp/include/raft/random/detail/rmat_rectangular_generator.cuh#L160-L162 irrespective of the random value that shall determine which bit to use for `dst_id`.

This PR refactors the loop. This way the `dst_id` number has the desired random distribution for all bits.

Authors:
  - Tamas Bela Feher (https://github.com/tfeher)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #2524

cpp/include/raft/random/detail/rmat_rectangular_generator.cuh

@@ -151,15 +151,16 @@ RAFT_KERNEL rmat_gen_kernel(IdxT* out,
   raft::random::PCGenerator gen{r.seed, r.base_subsequence + idx, 0};
   auto min_scale = min(r_scale, c_scale);
   IdxT i         = 0;
-  for (; i < min_scale; ++i) {
-    gen_and_update_bits(src_id, dst_id, a, a + b, a + b + c, r_scale, c_scale, i, gen);
-  }
-  for (; i < r_scale; ++i) {
-    gen_and_update_bits(src_id, dst_id, a + b, a + b, ProbT(1), r_scale, c_scale, i, gen);
-  }
-  for (; i < c_scale; ++i) {
-    gen_and_update_bits(src_id, dst_id, a + c, ProbT(1), ProbT(1), r_scale, c_scale, i, gen);
+  // Whether we have more rows than columns.
+  const bool more_rows = r_scale > c_scale;
+
+  for (; i < max_scale; ++i) {
+    ProbT A   = (i < min_scale) ? a : (more_rows ? a + b : a + c);
+    ProbT AB  = (i < min_scale) ? a + b : (more_rows ? a + b : ProbT(1));
+    ProbT ABC = (i < min_scale) ? a + b + c : ProbT(1);
+    gen_and_update_bits(src_id, dst_id, A, AB, ABC, r_scale, c_scale, i, gen);
   }
+
   store_ids(out, out_src, out_dst, src_id, dst_id, idx, n_edges);
 }