hasvk/elk: stop turning load_push_constants into load_uniform

Those intrinsics have different semantics in particular with regards
to divergence. Turning one into the other without invalidating the
divergence information breaks NIR validation. But also the conversion
means we get artificially less convergent values in the shaders.

So just handle load_push_constants in the backend and stop changing
things in Hasvk.

Fixes a bunch of tests in
   dEQP-VK.descriptor_indexing.*
   dEQP-VK.pipeline.*.push_constant.graphics_pipeline.dynamic_index_*

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34546>
3 files changed