My take on https://reviews.llvm.org/D83464
Basically it appears that in cases where we have noreturns or calls in an outliner candidate that has no LR, no available free regs, or any use of SP that we end up hitting stack fixup code with the MachineOutlinerDefault set as the FrameID. This triggers an assert of OF.FrameConstructionID != MachineOutlinerDefault && "Can only fix up stack references once" at llvm/lib/Target/AArch64/AArch64InstrInfo.cpp:6595 because a lot of the fixup code is not tested to handle fixing up more than once and needs some better checks for possible illegal code generated.
This diff has test cases to cover cases where you have noreturn functions as well as cases where you can hit this same behavior with a BLR terminator in your outliner candidate (but not have that counted as a thunk).
I have created a new diff instead of appending to D83464 because Phab wont let me.