[AVR] Fix a lowering bug in AVRISelLowering.cpp

The parseFunctionArgs() method was directly reading the
arguments from a Function object, but is should have used the
arguments supplied by the SelectionDAGBuilder.

This was causing
the lowering code to only lower one argument, not two in some cases.

Thanks to @brainlag on GitHub for coming up with the working fix!

Patch-by: @brainlag on GitHub
llvm-svn: 325474