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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301677 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed