[AMDGPU][GlobalISel] 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 guaranteed
to come to the same point at the same time.

This is the same optimization that was implemented for SelectionDAG in
D31731.

Differential Revision: https://reviews.llvm.org/D86609