nir/lower_subgroups: Don't do multiple lowerings at once

nir/lower_subgroups: Don't do multiple lowerings at once

Since using nir_shader_lower_instructions(), instructions get revisited
before proceeding with the next one. This already guarantees that any
subsequent lowerings of those instructions happen during the same pass
of nir_lower_subgroups().

v2: use nir_shader_lower_instructions() instead of setting the cursor.

Co-authored-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25123>