intel/compiler: Handle per-primitive inputs in FS

In Fragment Shader, regular inputs are laid out in the thread payload
in a one dword per each half-GRF, that gives room for having the two
delta dwords needed for interpolation.

Per-primitive inputs are laid out before the regular inputs, and since
there's no need to have delta information, they are packed.  So
half-GRF will be fully filled with 4 dwords of input.

When num_per_primitive_inputs is zero (the default case), behavior
should be the same as before.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13661>

src/intel/compiler/brw_fs.h

@@ -332,6 +332,7 @@ public:
    fs_reg get_timestamp(const brw::fs_builder &bld);
 
    fs_reg interp_reg(int location, int channel);
+   fs_reg per_primitive_reg(int location);
 
    virtual void dump_instructions() const;
    virtual void dump_instructions(const char *name) const;