i965/nir: lower TES PatchVerticesIn to a constant when a TCS is present

i965/nir: lower TES PatchVerticesIn to a constant when a TCS is present

When a TCS is present at link time we know the number of vertices in the
patch and we can lower gl_PatchVerticesIn in the TesEval stage directly
to a constant. We already have a pass for this that we use in the
Vulkan pipeline, so we just reuse that.

Notice that the GLSL linker also implements this optimization, which
we are not removing because other drivers may still depend on it, so
this should only be useful for OpenGL SPIR-V shaders for now.

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