[AVR] Add the AVRMCInstLower class

Summary:
This class deals with the lowering of CodeGen `MachineInstr` objects to
MC `MCInst` objects.

Reviewers: kparzysz, arsenm

Subscribers: wdng, beanz, japaric, mgorny

Differential Revision: https://reviews.llvm.org/D25269

llvm-svn: 283522