nir: Delete the nir_function_impl::start_block field.

nir: Delete the nir_function_impl::start_block field.

It's simply the first nir_cf_node in the nir_function_impl::body list,
which is easy enough to access - we don't to store a pointer to it
explicitly.  Removing it means we don't need to maintain the pointer
when, say, splitting the start block when modifying control flow.

Thanks to Connor Abbott for suggesting this.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>