intel/nir: Stop using nir_lower_vars_to_scratch

intel/nir: Stop using nir_lower_vars_to_scratch

Instead, we do a limited indirect deref lowering and then use
nir_lower_vars_to_explicit_types and nir_lower_explicit_io to lower it
as if it were SSBO or global memory access.  Among other things, this
should enable pointer arithmetic on local variables.  Fun!

The only shader-db change from this change on ICL was a few tiny cycle
count changes in 7 Aztec Ruins compute shaders.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5909>