[AVR] Fix codegen for rotate instructions

Summary:
    This patch introduces the ROLBRd and RORBRd pseudo-instructions,
    which implemenent the "traditional" rotate operations; instead of
    the AVR rotate instructions that use the carry bit.

    The code is not optimized at all. Especially when dealing with
    loops of rotate instructions, this codegen should be improved some
    day.

Related bug: 41358 <https://bugs.llvm.org/show_bug.cgi?id=41358>

//Note//: This is my first submitted patch.

Reviewers: dylanmckay, Jim

Reviewed By: dylanmckay

Subscribers: hiraditya, llvm-commits, dylanmckay, dsprenkels

Tags: #llvm

Patched by dsprenkels (Daan Sprenkels)

Differential Revision: https://reviews.llvm.org/D60365