intel/compiler: Use nir SUBGROUP_INVOCATION for RT TOPOLOGY_ID

This seems simpler, and made the sim happy (albeit in a case that doesn't impact Mesa today).

As a tangent, the sim was also happy with the codegen after:

-            ubld8.ADD(quarter(tmp, 1), quarter(tmp, 0), brw_imm_ud(8));
+            ubld8.ADD(quarter(tmp, 1), quarter(tmp, 0), brw_imm_uw(8));

This more closely matches the code from 8f48d23e, which after 6132992c is used to build SYSTEM_VALUE_SUBGROUP_INVOCATION.

Tested in ci: https://mesa-ci.01.org/jljusten/builds/371/group/63a9f0ea7bb98050796b649e85481845

Cc: @llandwerlin, @gfxstrand