coll/tuned: don't select algorithms knowing when it's clear they would fall back to linear

Bcast: scatter_allgather and scatter_allgather_ring expect N_elem >= N_procs
Allreduce: rabenseifner expects N_elem >= pow2 nearest to N_procs

In all cases, the implementations will fall back to a linear implementation,
which will most likely yield the worst performance (noted for 4B bcast on 128 ranks)

Signed-off-by: Joseph Schuchart <schuchart@icl.utk.edu>

ompi/mca/coll/tuned/coll_tuned_decision_fixed.c

@@ -3,7 +3,7 @@
  * Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
  *                         University Research and Technology
  *                         Corporation.  All rights reserved.
- * Copyright (c) 2004-2015 The University of Tennessee and The University
+ * Copyright (c) 2004-2020 The University of Tennessee and The University
  *                         of Tennessee Research Foundation.  All rights
  *                         reserved.
  * Copyright (c) 2004-2005 High Performance Computing Center Stuttgart,
@@ -567,9 +567,7 @@ int ompi_coll_tuned_bcast_intra_dec_fixed(void *buff, int count,
      *  {9, "scatter_allgather_ring"},
      */
     if (communicator_size < 4) {
-        if (total_dsize < 2) {
+        if (total_dsize < 32) {
-            alg = 9;
-        } else if (total_dsize < 32) {
             alg = 3;
         } else if (total_dsize < 256) {
             alg = 5;
@@ -591,9 +589,7 @@ int ompi_coll_tuned_bcast_intra_dec_fixed(void *buff, int count,
             alg = 5;
         }
     } else if (communicator_size < 8) {
-        if (total_dsize < 2) {
+        if (total_dsize < 64) {
-            alg = 8;
-        } else if (total_dsize < 64) {
             alg = 5;
         } else if (total_dsize < 128) {
             alg = 6;
@@ -639,8 +635,6 @@ int ompi_coll_tuned_bcast_intra_dec_fixed(void *buff, int count,
     } else if (communicator_size < 256) {
         if (total_dsize < 2) {
             alg = 6;
-        } else if (total_dsize < 128) {
-            alg = 8;
         } else if (total_dsize < 16384) {
             alg = 5;
         } else if (total_dsize < 32768) {