llvmpipe interpolation and sample mask changes for ARB_gpu_shader5
This is two more of the pieces of ARB_gpu_shader5 support.
The MR contain:
- a sample mask fix
- removal of non-simple attrib interp code
- 2 interpolation fixes
- refactor for some of the interpolation code
- support for ARB_gpu_shader5 interpolateAt* instructions in gallivm/llvmpipe
- support for gl_SampleMaskIn