[Assignment Tracking][SelectionDAG] Downgrade dbg.assigns to dbg.values if...

[Assignment Tracking][SelectionDAG] Downgrade dbg.assigns to dbg.values if assignment tracking is not enabled

We shouldn't be able to reach this code path from source code but this provides
a better fail-safe than asserting. The result of the downgrade is a degraded
debugging experience, but it is better than nothing.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D148212