[AVR] Fix lifeness issues in the AVR backend

This patch is a large number of small changes that should hopefully not
affect the generated machine code but are still important to get right
so that the machine verifier won't complain about them.

The llvm/test/CodeGen/AVR/pseudo/*.mir changes are also necessary
because without the liveins the used registers are considered undefined
by the machine verifier and it will complain about them.

Differential Revision: https://reviews.llvm.org/D97172