[AVR] Fix rotate instructions

This patch fixes some issues with the RORB pseudo instruction.

  - A minor issue in which the instructions were said to use the SREG,
    which is not true.
  - An issue with the BLD instruction, which did not have an output operand.
  - A major issue in which invalid instructions were generated. The fix
    also reduce RORB from 4 to 3 instructions, so it's also a small
    optimization.

These issues were flagged by the machine verifier.

Differential Revision: https://reviews.llvm.org/D96957