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.