[SelectionDAG] Pass std::vector by reference instead of by pointer to BuildSDIV/Build...

[SelectionDAG] Pass std::vector by reference instead of by pointer to BuildSDIV/BuildUDIV.

This removes the need for an assert to ensure the pointer isn't null.

Years ago we had ifs the checked the pointer was non-null before very access to the vector. These checks were removed and replaced with a single assert. But a reference seems more suitable here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338205 91177308-0d34-0410-b5e6-96231b3b80d8