This adds the majority of the AVR backend.
LGTM with some style nits, plus the pass setup seems to be adding a couple of passes I don't see being added
C++ style comments
Why not all caps like the others?
I think it's best to not duplicate the comments on overrides. The base class header comment is more likely to be kept up to date
brace on case lines
Some comments about why these registers are reserved might be helpful
C++ comments. Repeat for everywhere else
Do these actually initialize to 0? I don't think I've seen empty parens like this before
return on new line like the others here
This is only one place, so you can just move the literal there
No return after else
Don't see this
I don't see this file?