AMDGPU/SI: Add llvm.amdgcn.mbcnt.* intrinsics

Summary:
These are meant to be used instead of the llvm.SI.tid intrinsic which will
be deprecated at some point.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D15475

llvm-svn: 255652