Sparse multi-map from virtual registers to their local SUnit uses.
Declared in <llvm/CodeGen/ScheduleDAGInstrs.h>
using VReg2SUnitMultiMap = SparseMultiSet<VReg2SUnit, Register, VirtReg2IndexFunctor>;These uses are gathered by the DAG builder and may be consulted by the scheduler to avoid iterating an entire vreg use list.