intel/fs: make tcs input_vertices dynamic

intel/fs: make tcs input_vertices dynamic

We need to do 3 things to accomplish this :

   1. make all the register access consider the maximal case when
      unknown at compile time

   2. move the clamping of load_per_vertex_input prior to lowering
      nir_intrinsic_load_patch_vertices_in (in the dynamic cases, the
      clamping will use the nir_intrinsic_load_patch_vertices_in to
      clamp), meaning clamping using derefs rather than lowered
      nir_intrinsic_load_per_vertex_input

   3. in the known cases, lower nir_intrinsic_load_patch_vertices_in
      in NIR (so that the clamped elements still be vectorized to the
      smallest number of URB read messages)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22378>