AMDGPU/GlobalISel: Add support for amdgpu_vs calling convention

Reviewers: arsenm

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D35916

llvm-svn: 309675