i965: Handle TCS outputs and TES inputs.

i965: Handle TCS outputs and TES inputs.

TCS outputs and TES inputs both refer to a common "patch URB entry"
shared across all invocations.  First, there are some number of
per-patch entries.  Then, there are per-vertex entries accessed via
an offset for the variable and a stride times the vertex index.

Because these calculations need to be done in both the vec4 and scalar
backends, it's simpler to just compute the offset calculations in NIR.
It doesn't necessarily make much sense to use per-vertex intrinsics
afterwards, but that at least means we don't lose the per-patch vs.
per-vertex information.

v2: Use is_input/is_output helpers (suggested by Jordan Justen).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>