nir: Add new texop nir_texop_tex_prefetch

This is like nir_texop_tex, but signals that the sampling coordinates
are immutable during the shader stage, in a way that allows the HW
that supports pre-dispatching sampling operations to pre-fetch
the result prior to scheduling the shader stage.

This is introduced to support the feature in Freedreno. Adreno HW
from a4xx supports it.

A NIR pass introduced later in this series will detect sampling
operations that are eligible for pre-dispatch, and replace
nir_texop_tex by this new op, to tell the backend to enable
pre-fetch.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>

src/compiler/nir/nir.h

@@ -1736,6 +1736,7 @@ typedef enum {
    nir_texop_samples_identical,  /**< Query whether all samples are definitely
                                   * identical.
                                   */
+   nir_texop_tex_prefetch,       /**< Regular texture look-up, eligible for pre-dispatch */
 } nir_texop;
 
 typedef struct {