nir/lower_tex: Add a way to lower TXS(non-0-LOD) instructions

The V3D driver has an open-coded solution for this, and we need the
same thing for Panfrost, so let's add a generic way to lower TXS(LOD)
into max(TXS(0) >> LOD, 1).

Changes in v2:
* Use == 0 instead of !
* Rework the minification logic as suggested by Jason
* Assign cursor pos at the beginning of the function
* Patch the LOD just after retrieving the old value

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

src/compiler/nir/nir.h

@@ -3426,6 +3426,12 @@ typedef struct nir_lower_tex_options {
     */
    bool lower_txd_clamp_if_sampler_index_not_lt_16;
 
+   /**
+    * If true, lower nir_texop_txs with a non-0-lod into nir_texop_txs with
+    * 0-lod followed by a nir_ishr.
+    */
+   bool lower_txs_lod;
+
    /**
     * If true, apply a .bagr swizzle on tg4 results to handle Broadcom's
     * mixed-up tg4 locations.