freedreno/ir3: Lower GS builtins before lowering IO

freedreno/ir3: Lower GS builtins before lowering IO

We mostly got away with replacing a store_output with a store_var, but
for complex types like structs, that doesn't work. Once the IO has
been lowered from vars to intrinsic, we've lost the deref chains and
can't properly shadow the outputs.

This commits moves the GS lowering up so we do it before the output
variables get lowered to store_output.  This way the pass works much
like nir_lower_io_to_temporaries() and cleanly shadows the outputs.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4562>