AMDGPU/SI: Add amdgpu_kernel calling convention. Part 2.

    Summary:
    Check calling convention in AMDGPUMachineFunction::isKernel

    This will be used for AMDGPU_HSA_KERNEL symbol type in output ELF.

    Also, in the future unused non-kernels may be optimized.

    Reviewers: tstellarAMD, arsenm

    Subscribers: arsenm, joker.eph, llvm-commits

    Differential Revision: http://reviews.llvm.org/D19917

llvm-svn: 268719