AMDGPU: Add llvm.amdgcn.exp2 intrinsic

Provide direct access to v_exp_f32 and v_exp_f16, so we can start
correctly lowering the generic exp intrinsics.

Unfortunately have to break from the usual naming convention of
matching the instruction name and stripping the v_ prefix. exp is
already taken by the export intrinsic. On the clang builtin side, we
have a choice of maintaining the convention to the instruction name,
or following the intrinsic name.