AMDGPU/SI: Add llvm.amdgcn.s.waitcnt.all intrinsic

Summary:
So it appears that to guarantee some of the ordering requirements of a GLSL
memoryBarrier() executed in the shader, we need to emit an s_waitcnt.

(We can't use an s_barrier, because memoryBarrier() may appear anywhere in
the shader, in particular it may appear in non-uniform control flow.)

Reviewers: arsenm, mareko, tstellarAMD

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D19203

llvm-svn: 267729