I bisected this to LLVM r182062: 'Generate debug info for by-value struct args
even if they are not used.' Enabling the DEBUG code in
SelectionDAGISel::LowerArguments(), it complains that "LowerFormalArguments
emitted a null value!".

I suspect SITargetLowering::LowerFormalArguments() needs to be adapted for
this, but I'm not sure how. Christian, can you take a look?