AMDGPU: Add skeleton GlobalIsel implementation

Summary:
This adds the necessary target code to be able to run the ir translator.
Lowering function arguments and returns is a nop and there is no support
for RegBankSelect.

Reviewers: arsenm, qcolombet

Subscribers: arsenm, joker.eph, vkalintiris, llvm-commits

Differential Revision: http://reviews.llvm.org/D19077

llvm-svn: 266356