[AVR] Set trackLivenessAfterRegAlloc

This sets trackLivenessAfterRegAlloc on AVRRegisterInfo.

Most existing targets set this flag. Without it, specific IR inputs
cause LLVM to fail with:

Assertion failed: (getParent()->getProperties().hasProperty( MachineFunctionProperties::Property::TracksLiveness) &&
                   "Liveness information is accurate"), function livein_begin
file MachineBasicBlock.cpp, line 1354.

With this commit, this no longer happens.

Patch by Peter Nimmervoll.

llvm-svn: 334409