[DebugInfo] Correctly track SDNode dependencies for list debug values

During SelectionDAG, we must track the SDNodes that each SDDbgValue depends on
to compute its value. These are ultimately derived from the location operands to
the SDDbgValue, but were stored in a separate vector prior to this patch. This
resulted in cases where one of the lists was updated incorrectly, resulting in
crashes during compilation. This patch fixes the issue by directly recomputing
the dependency list from the SDDbgOperands in getDependencies().

Differential Revision: https://reviews.llvm.org/D99423