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 7ca26c5fa2df253878cab22e1e2f0d6f1b481218.