nir/lower_io: Make get_io_offset() return a nir_ssa_def * for indirects.

nir/lower_io: Make get_io_offset() return a nir_ssa_def * for indirects.

get_io_offset() already walks the dereference chain and discovers
whether or not we have an indirect; we can just return that rather than
computing it a second time via deref_has_indirect().  This means moving
the call a bit earlier.

By returning a nir_ssa_def *, we can pass back both an existence flag
(via NULL checking the pointer) and the value in one parameter.  It
also simplifies the code somewhat.  nir_lower_samplers works in a
similar fashion.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>