intel/fs: Use image_deref intrinsics instead of image_var

Since we had to rewrite the deref walking loop anyway, I took the
opportunity to make it a bit clearer and more efficient.  In particular,
in the AoA case, we will now emit one minmax instead of one per array
level.

Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_fs.h

@@ -235,7 +235,7 @@ public:
    fs_reg get_nir_src(const nir_src &src);
    fs_reg get_nir_src_imm(const nir_src &src);
    fs_reg get_nir_dest(const nir_dest &dest);
-   fs_reg get_nir_image_deref(const nir_deref_var *deref);
+   fs_reg get_nir_image_deref(nir_deref_instr *deref);
    fs_reg get_indirect_offset(nir_intrinsic_instr *instr);
    void emit_percomp(const brw::fs_builder &bld, const fs_inst &inst,
                      unsigned wr_mask);