[AVR] Add instruction selection lowering code

Summary: This adds AVRISelLowering.cpp

Reviewers: kparzysz, arsenm

Subscribers: wdng, beanz, mgorny

Differential Revision: https://reviews.llvm.org/D25034

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282711 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed