[Dominators] Change Roots type to SmallVector

Summary: We can use the template parameter `IsPostDom` to pick an appropriate SmallVector size to store DomTree roots for dominators and postdominators. Before, the code would always allocate memory with `std::vector`.

Reviewers: dberlin, davide, sanjoy, grosser

Reviewed By: grosser

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35636

llvm-svn: 309148