[SelectionDAG] Don't pass a scalable vector to MachinePointerInfo::getWithOffset in a unit test.

Suppresses an implicit TypeSize to uint64_t conversion warning.

We might be able to just not offset it since we're writing to a
Fixed stack object, but I wasn't sure so I just did what
DAGTypeLegalizer::IncrementPointer does.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D98736