nir: Fix divergence analysis for tessellation input/outputs

nir: Fix divergence analysis for tessellation input/outputs

The load_per_vertex_{input,output} intrinsics simply mean that they're
reading an arrayed input/output, which have one element per invocation.
Most accesses to those use gl_InvocationID as the subscript.  However,
it's totally possible to read any element of the array.  For example,
an evaluation shader might read gl_in[2].gl_Position, or a control
shader might read output[0].

For threads processing a single patch, an input/output load is
convergent if and only if both sources (the per-vertex-array subscript
and the offset) are convergent.  For threads processing multiple
patches, we continued to mark them divergent.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5613>