[AVR] Disassemble double register instructions

Add disassembly support for the movw, adiw, and sbiw instructions.

I had previously committed test cases for the adiw and sbiw
instructions, but had accidentally made them not runnable so they were
skipped all this time. Oops. This patch fixes that by adding support for
disassembling those instructions.

Differential Revision: https://reviews.llvm.org/D82093