AMDGPU: Add new intrinsic llvm.amdgcn.kill(i1)

Summary:
Kill the thread if operand 0 == false.
llvm.amdgcn.wqm.vote can be applied to the operand.

Also allow kill in all shader stages.

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D38544

llvm-svn: 316427