[AMDGPU] Eliminate barrier if workgroup size is not greater than wavefront size

[AMDGPU] Eliminate barrier if workgroup size is not greater than wavefront size

If a workgroup size is known to be not greater than wavefront size
the s_barrier instruction is not needed since all threads are guarantied
to come to the same point at the same time.

Differential Revision: https://reviews.llvm.org/D31731

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299659 91177308-0d34-0410-b5e6-96231b3b80d8