intel/brw: Avoid modifying the shader in assign_curb_setup if not needed

If there are no uniforms to push, don't emit the AND or invalidate the
shader analysis.  This affects only compute shaders.

Not a significant impact since lots of shaders end up pushing
uniforms.  Fossil-db numbers (restricted to compute pipelines only) for DG2

```
Totals:
Instrs: 3071016 -> 3070894 (-0.00%)
Cycle count: 8320268863 -> 8320264519 (-0.00%)

Totals from 122 (2.70% of 4520) affected shaders:
Instrs: 10675 -> 10553 (-1.14%)
Cycle count: 2060003 -> 2055659 (-0.21%)
```

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30631>

src/intel/compiler/brw_fs.cpp

@@ -1117,7 +1117,7 @@ fs_visitor::assign_curb_setup()
        */
       assert(devinfo->verx10 >= 125);
       assert(uniform_push_length <= reg_unit(devinfo));
-   } else if (is_compute && devinfo->verx10 >= 125) {
+   } else if (is_compute && devinfo->verx10 >= 125 && uniform_push_length > 0) {
       assert(devinfo->has_lsc);
       fs_builder ubld = fs_builder(this, 1).exec_all().at(
          cfg->first_block(), cfg->first_block()->start());