[SelectionDAG] Salvage debug info for non-constant ADDs (#68981)

Teach SelectionDAG::salvageDebugInfo() to salvage debug information for
ADD nodes where the RHS is non-constant.
    
Co-authored-by: Mikael Holmen <mikael.holmen@ericsson.com>

- [DebugInfo] Precommit testcase for pointer addition with unknown
offset
- [SelectionDAG] Salvage debug info for non-constant ADDs

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Co-authored-by: Mikael Holmen <mikael.holmen@ericsson.com>