If you attach attribute((optnone)) to a function when using optimisations, that function will use fast-isel instead of the usual SelectionDAG method. This is a problem for instruction referencing, because it means DBG_VALUEs of virtual registers will be created, triggering some safety assertions in LiveDebugVariables. Those assertions exist to detect exactly this scenario, where an unexpected piece of code is generating virtual register references in instruction referencing mode.
Fix this by transforming the DBG_VALUEs created by fast-isel into half-formed DBG_INSTR_REFs, after which they get patched up in finalizeDebugInstrRefs. The test modified adds a fast-isel mode to the instruction referencing isel test.