Compiler related performance issues in target code generation

[sithhell]

This is supposed to be meta ticket to track compiler related performance problems in combination with the allscale runtime we identified so far.

• Compiler should not use treeture<void> in process variants #6 Compiler should not use treeture in process variants
• DataItemManager::get is not supposed be used in tight loops #7 DataItemManager::get is not supposed be used in tight loops
• Compiler should not use treeture<T> in process variants #9 Compiler should not use treeture in process variants

Please check the boxes once the fixes have landed on master and amend the list once new issues have been found.