i965/nir: add a helper to lower gl_PatchVerticesIn to a uniform

i965/nir: add a helper to lower gl_PatchVerticesIn to a uniform

v2: do not try to handle it as a system value directly for the SPIR-V
    path. In GL we rather handle it as a uniform like we do for the
    GLSL path (Jason).

v3:
  - Remove the uniform variable, it is alwats -1 now (Jason)
  - Also do the lowering for the TessEval stage (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>