coll: refactor barrier_intra_k_dissemination

Because the compiler can't figure out the arithmetics, it is warning:
    ‘MPIC_Waitall’ accessing 8 bytes in a region of size 0
[-Wstringop-overflow=]

Refactor to suppress warning and for better readability.

src/mpi/coll/barrier/barrier_intra_k_dissemination.c

@@ -84,6 +84,8 @@ int MPIR_Barrier_intra_k_dissemination(MPIR_Comm * comm, int coll_group, int k,
  nphases++;
  }
 
+ MPIR_Request **rreqs = recv_reqs;
+ MPIR_Request **prev_rreqs = recv_reqs + (k - 1);
  shift = 1;
  for (i = 0; i < nphases; i++) {
  for (j = 1; j < k; j++) {
@@ -95,14 +97,12 @@ int MPIR_Barrier_intra_k_dissemination(MPIR_Comm * comm, int coll_group, int k,
  MPIR_Assert(to >= 0 && to < nranks);
 
  /* recv from (k-1) nbrs */
- mpi_errno =
- MPIC_Irecv(NULL, 0, MPI_BYTE, from, MPIR_BARRIER_TAG, comm, coll_group,
- &recv_reqs[(j - 1) + ((k - 1) * (i & 1))]);
+ mpi_errno = MPIC_Irecv(NULL, 0, MPI_BYTE, from, MPIR_BARRIER_TAG, comm, coll_group,
+ &rreqs[j - 1]);
  MPIR_ERR_COLL_CHECKANDCONT(mpi_errno, errflag, mpi_errno_ret);
  /* wait on recvs from prev phase */
  if (i > 0 && j == 1) {
- mpi_errno =
- MPIC_Waitall(k - 1, &recv_reqs[((k - 1) * ((i - 1) & 1))], MPI_STATUSES_IGNORE);
+ mpi_errno = MPIC_Waitall(k - 1, prev_rreqs, MPI_STATUSES_IGNORE);
  MPIR_ERR_COLL_CHECKANDCONT(mpi_errno, errflag, mpi_errno_ret);
  }
 
@@ -114,10 +114,13 @@ int MPIR_Barrier_intra_k_dissemination(MPIR_Comm * comm, int coll_group, int k,
  mpi_errno = MPIC_Waitall(k - 1, send_reqs, MPI_STATUSES_IGNORE);
  MPIR_ERR_COLL_CHECKANDCONT(mpi_errno, errflag, mpi_errno_ret);
  shift *= k;
+
+ MPIR_Request **tmp = rreqs;
+ rreqs = prev_rreqs;
+ prev_rreqs = tmp;
  }
 
- mpi_errno =
- MPIC_Waitall(k - 1, recv_reqs + ((k - 1) * ((nphases - 1) & 1)), MPI_STATUSES_IGNORE);
+ mpi_errno = MPIC_Waitall(k - 1, prev_rreqs, MPI_STATUSES_IGNORE);
  MPIR_ERR_COLL_CHECKANDCONT(mpi_errno, errflag, mpi_errno_ret);
 
  fn_exit: