function,file sql_base.cc

When uncacheable queries are written to a temp table the optimizer must 
preserve the original JOIN structure, because it is re-using the JOIN 
structure to read from the resulting temporary table.
This was done only for uncacheable sub-queries. 
But top level queries can also benefit from this mechanism, specially if 
they're using index access and need a reset.
Fixed by not limiting the saving of JOIN structure to subqueries
exclusively.
Added a new test file to extend the existing (large) subquery.test.