Using FoldingSet in SelectionDAG::getVTList.

VTList has a long life cycle through the module and getVTList is frequently called. In current getVTList, sequential search over a std::vector is used, this is inefficient in big module.
This patch use FoldingSet to implement hashing mechanism when searching.

Reviewer: Nadav Rotem
Test    : Pass unit tests & LNT test suite