nir: Add option to lower tex to txl when shader don't support implicit LOD

We already add the LOD src, so go ahead and update the texop as well
when this option is set.

v2: Make it an option. (Rob Clark)

v3: Use a more concise name suggested by Jason.

Reviewed-by: Rob Clark <robdclark@gmail.com>

src/compiler/nir/nir.h

@@ -3264,6 +3264,12 @@ typedef struct nir_lower_tex_options {
     */
    unsigned lower_srgb;
 
+   /**
+    * If true, lower nir_texop_tex on shaders that doesn't support implicit
+    * LODs to nir_texop_txl.
+    */
+   bool lower_tex_without_implicit_lod;
+
    /**
     * If true, lower nir_texop_txd on cube maps with nir_texop_txl.
     */

src/compiler/nir/nir_lower_tex.c

@@ -1127,6 +1127,8 @@ nir_lower_tex_block(nir_block *block, nir_builder *b,
            (tex->op == nir_texop_tex && !shader_supports_implicit_lod))) {
          b->cursor = nir_before_instr(&tex->instr);
          nir_tex_instr_add_src(tex, nir_tex_src_lod, nir_src_for_ssa(nir_imm_int(b, 0)));
+         if (tex->op == nir_texop_tex && options->lower_tex_without_implicit_lod)
+            tex->op = nir_texop_txl;
          progress = true;
          continue;
       }