R600: Initialize AMDGPUMachineFunction::ShaderType to ShaderType::COMPUTE

We need to intialize this to something and since clang does not set
the shader type attribute and clang is used only for compute shaders,
initializing it to COMPUTE seems like the best choice.

Reviewed-by: Christian König <christian.koenig@amd.com>
llvm-svn: 180620