intel/elk: Move wpos_w setup right into nir_intrinsic_load_frag_w.

Given that the intrinsic will be CSEed at the NIR level, we don't need to
preemptively set it up at the top of the shader.  No change in HSW shader-db.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25190>

src/intel/compiler/elk/elk_fs.h

@@ -392,7 +392,6 @@ public:
    elk_fs_reg uw_pixel_x;
    elk_fs_reg uw_pixel_y;
    elk_fs_reg pixel_z;
-   elk_fs_reg wpos_w;
    elk_fs_reg delta_xy[ELK_BARYCENTRIC_MODE_COUNT];
    elk_fs_reg final_gs_vertex_count;
    elk_fs_reg control_data_bits;

src/intel/compiler/elk/elk_fs_nir.cpp

@@ -3926,7 +3926,7 @@ fs_nir_emit_fs_intrinsic(nir_to_elk_state &ntb,
    case nir_intrinsic_load_frag_coord_w:
       /* Lowered to interpolation pre-gen6. */
       assert(devinfo->ver >= 6);
-      bld.MOV(dest, s.wpos_w);
+      bld.emit(ELK_SHADER_OPCODE_RCP, dest, fetch_payload_reg(bld, s.fs_payload().source_w_reg));
       break;
 
    case nir_intrinsic_load_interpolated_input: {

src/intel/compiler/elk/elk_fs_visitor.cpp

@@ -140,12 +140,6 @@ elk_fs_visitor::emit_interpolation_setup_gfx4()
       this->delta_xy[ELK_BARYCENTRIC_PERSPECTIVE_PIXEL];
 
    abld = bld.annotate("compute pos.w and 1/pos.w");
-   /* Compute wpos.w.  It's always in our setup, since it's needed to
-    * interpolate the other attributes.
-    */
-   this->wpos_w = vgrf(glsl_float_type());
-   abld.emit(ELK_FS_OPCODE_LINTERP, wpos_w, delta_xy,
-             interp_reg(abld, VARYING_SLOT_POS, 3, 0));
 }
 
 /** Emits the interpolation for the varying inputs. */
@@ -273,13 +267,6 @@ elk_fs_visitor::emit_interpolation_setup_gfx6()
    if (wm_prog_data->uses_src_depth)
       this->pixel_z = fetch_payload_reg(bld, fs_payload().source_depth_reg);
 
-   if (wm_prog_data->uses_src_w) {
-      abld = bld.annotate("compute pos.w");
-      this->wpos_w = vgrf(glsl_float_type());
-      abld.emit(ELK_SHADER_OPCODE_RCP, this->wpos_w,
-                fetch_payload_reg(abld, fs_payload().source_w_reg));
-   }
-
    if (wm_key->persample_interp == ELK_SOMETIMES) {
       assert(!elk_needs_unlit_centroid_workaround(devinfo));