AMDGPU: Add new amdgcn.init.exec intrinsics

v2: More tests, bug fixes, cosmetic changes.

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, llvm-commits, t-tye

Differential Revision: https://reviews.llvm.org/D31762

llvm-svn: 301677