nir/lower_system_values: simplify load_helper_invocation lowering

The two backends (ir3, dxil) using the lowering have
info->fs.uses_sample_shading matching set when sample shading is used -- the VK
drivers pass the rasterization state flag into the compiler, while
freedreno and d3d12 have caps->force_persample_interp so the frontend
creates a shader variant with info->fs.uses_sample_shading set.

This means that we can drop the sample-id SHL/AND in the pixel-rate
shading case, which in turn means that drivers don't need to have a
load_sample_id() that doesn't trigger sample-rate shading (which Adreno
doesn't have).

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36429>
2 files changed