AMDGPU/GlobalISel: Don't try to lower geometry shaders

Summary: The AMDGPU_GS calling convention is not supported yet.

Reviewers: arsenm, nhaehnle

Reviewed By: nhaehnle

Subscribers: kzhuravl, wdng, yaxunl, rovka, kristof.beyls, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D46041

llvm-svn: 331186