[AMDGPU] Expose flat work group size, register and wave control attributes

__attribute__((amdgpu_flat_work_group_size(<min>, <max>))) - request minimum and maximum flat work group size
__attribute__((amdgpu_waves_per_eu(<min>[, <max>]))) - request minimum and/or maximum waves per execution unit

Differential Revision: https://reviews.llvm.org/D24513

llvm-svn: 282371