Reapply "[DebugInfo] Fix updateDbgUsersToReg to support DBG_VALUE_LIST"

Previous crashes caused by this patch were the result of machine
subregisters being incorrectly handled in updateDbgUsersToReg; this has
been fixed by using RegUnits to determine overlapping registers, instead
of using the register values directly.

Differential Revision: https://reviews.llvm.org/D101523

This reverts commit 7ca26c5f.