account that "FORCE INDEX" disables full table scans, and not range/index_merge scans.
(with post-review fixes)


mysql-test/r/index_merge.result:
  Testcase for BUG#16166
mysql-test/t/index_merge.test:
  Testcase for BUG#16166
sql/sql_select.cc:
  BUG#16166: "Can't use index_merge with FORCE INDEX": adjust the heuristics check:
  if (force-index-is-used && there-is-possible-ref-access && 
  +    THERE IS NO POSSIBLE RANGE/INDEX_MERGE ACCESS)
  {
    ...