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_print.c

@@ -985,6 +985,9 @@ print_tex_instr(nir_tex_instr *instr, print_state *state)
    case nir_texop_samples_identical:
       fprintf(fp, "samples_identical ");
       break;
+   case nir_texop_tex_prefetch:
+      fprintf(fp, "tex (pre-dispatchable) ");
+      break;
    default:
       unreachable("Invalid texture operation");
       break;