[AVR] Fix useDeprecatedPositionallyEncodedOperands errors.

This is a follow-on to https://reviews.llvm.org/D134073.

It renames a few fields to have consistent names, as well as renaming
operands to match the field names.

The encoder behavior is unchanged by this cleanup, but a few
instructions were previously being disassembled incorrectly, and have
been corrected by this change. All of the affected instructions were
missing disassembly tests, which are now added.

Differential Revision: https://reviews.llvm.org/D134185