Bug#48623: Multiple subqueries are optimized incorrectly
      
The function setup_semijoin_dups_elimination() has a major loop that
goes through every table in the JOIN object. Usually, there is a normal
"plus one" increment in the for loop that implements this, but each semijoin
nest is treated as one entity and there is another increment that skips past
the semijoin nest to the next table in the JOIN object. However, when
combining these two increments, the next joined table is skipped, and if that
happens to be the start of another semijoin nest, the correct processing
for that nest will not be carried out.

mysql-test/r/subselect_sj.result:
  Added test results for bug#48623
mysql-test/r/subselect_sj_jcl6.result:
  Added test results for bug#48623
mysql-test/t/subselect_sj.test:
  Added test case for bug#48623
sql/opt_subselect.cc:
  Omitted the "plus one" increment in the for loop, added "plus one"
  in the remaining switch case, fixed coding style issue in remaining
  increment operations.