Fix two problems:
1. Let optimize_semijoin_nests() reset sj_nest->sjmat_info irrespectively  
   of value of optimizer_flag. We need this in case somebody has turned optimization
   off between reexecutions of the same statement.

2. Do not pull out constant tables out of semi-join nests. The problem is that pullout 
   operation is not undoable, and if a table is constant because it is 1/0-row table it 
   may cease to be constant on the next execution. Note that tables that are constant
   because of possible eq_ref(const) access will still be pulled out as they are
   considered functionally-dependent.