[AVR] Support STDSPQRr on reduced tiny cores (#230226)
Currently we always expand `STDSPQRr` to `STDPtrQRr`: which is wrong, because
`STDPtrQRr` doesn't exist on Tiny. This merge request fixes this expansion, same
way we already have `if (STI.hasTinyEncoding()) { ...}` for several other instructions.
Closes https://github.com/llvm/llvm-project/issues/226680.
