[Assignment Tracking][SelectionDAG] Fix dbg.declare location nullptr deref

Debug intrinsics sometimes end up with empty metadata location operands. The
debug intrinsic interfaces return nullptr when retrieving location operand in
this case.

When assignment tracking is not enabled a dbg.declare with a nullptr location
operand is skipped. Do the same when assignment tracking is enabled (a nullptr
address component of a dbg.assign is already handled correctly.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D148203