[AVR] Do not use R0/R1 on avrtiny

This patch makes sure the compiler uses R16/R17 on avrtiny (attiny10
etc) instead of R0/R1.

Some notes:

  * For the NEGW and ROLB instructions, it adds an explicit zero
    register. This is necessary because the zero register is different
    on avrtiny (and InstrInfo Uses lines need a fixed register).
  * Not entirely sure about putting all tests in features/avr-tiny.ll,
    but it doesn't seem like the "target-cpu"="attiny10" attribute
    works.

Updates: https://github.com/llvm/llvm-project/issues/53459

Differential Revision: https://reviews.llvm.org/D138582