[AMDGPU] New llvm.amdgcn.ballot intrinsic

Add a new llvm.amdgcn.ballot intrinsic modeled on the ballot function
in GLSL and other shader languages. It returns a bitfield containing the
result of its boolean argument in all active lanes, and zero in all
inactive lanes.

This is intended to replace the existing llvm.amdgcn.icmp and
llvm.amdgcn.fcmp intrinsics after a suitable transition period.

Use the new intrinsic in the atomic optimizer pass.

Differential Revision: https://reviews.llvm.org/D65088