Thank you Hal, I removed the unused variable.

I've fixed the coding style issue in Nios2InstrInfo::copyPhysReg.

Thanks for another comment Hal, I've updated the test accordingly.

Thanks for the review Hal. When I was addressing the things that you pointed out I found that entire analyzeReturn method is already implemented in LLVM, which allowed me to remove some code.

Added R2 run to LIT tests, fixed a typo with unintended comment.

Thanks Akira for your comments, I made all changes that you suggested.

I added test for driver to check if target and CPU flags are recognized, as Craig suggested.

I implemented changes suggested by Joerg.

Additionally this information may be useful in context of this patch: here is specification of Nios2 builtins as implemented in GCC: https://gcc.gnu.org/onlinedocs/gcc/Altera-Nios-II-Built-in-Functions.html