nir: add nir_lower_amul pass

Lower amul to either imul or imul24, depending on whether 24b is enough
bits to calculate an offset within the thing being dereferenced.

Signed-off-by: Rob Clark <robdclark@chromium.org>

src/compiler/nir/nir.h

@@ -2757,6 +2757,14 @@ typedef struct nir_shader_compiler_options {
    /* Lowers when rotate instruction is not supported */
    bool lower_rotate;
 
+   /**
+    * Backend supports imul24, and would like to use it (when possible)
+    * for address/offset calculation.  If true, driver should call
+    * nir_lower_amul().  (If not set, amul will automatically be lowered
+    * to imul.)
+    */
+   bool has_imul24;
+
    /**
     * Is this the Intel vec4 backend?
     *
@@ -3540,6 +3548,8 @@ void nir_compact_varyings(nir_shader *producer, nir_shader *consumer,
 void nir_link_xfb_varyings(nir_shader *producer, nir_shader *consumer);
 bool nir_link_opt_varyings(nir_shader *producer, nir_shader *consumer);
 
+bool nir_lower_amul(nir_shader *shader,
+                    int (*type_size)(const struct glsl_type *, bool));
 
 void nir_assign_io_var_locations(struct exec_list *var_list,
                                  unsigned *size,