cse_main: Check for used symbols, workaround for #5331
Add test
Fix cse with iterator

failing test_multiple_expressions
symbols are lookuped up for reduced_exprs.
Cleanup whitespace
Simplified test_name_conflict
Add a function to filter out excluded symbols and test.
Use filter_symbols in cse and add test.
Merge pull request #7458 from lennart0901/workaround_5331

cse_main: Check for used symbols, workaround for #5331