In the InstrRefBasedImpl for LiveDebugValues, we attempt to propagate debug values through basic blocks in part by checking to see whether all a variable's incoming debug values to a BB "agree", i.e. whether their properties match and they refer to the same underlying value.
Prior to this patch, the check for agreement between incoming values relied on exact equality, which meant that a VPHI and a Def DbgValue that referred to the same underlying value would be seen as disagreeing. This patch changes this behaviour to treat them as referring to the same value, allowing the shared value to propagate into the BB.