zink: adjust swizzled deref loads by the variable component offset

this code is intended to transform a block like:

```
  32    %306 = @load_interpolated_input (%34, %0 (0x0)) (base=3, component=2, dest_type=float32, io location=VARYING_SLOT_VAR3 slots=1)
  32x2  %307 = fsub %305, %306.xx
```

into derefs. the existing code generates this:

```
decl_var shader_in INTERP_MODE_NONE none vec2 #7 (VARYING_SLOT_VAR3.zw, 0, 0)
  32    %516 = deref_var &#7 (shader_in vec2)
  32x2  %517 = @load_deref (%516) (access=none)
  32    %518 = mov %517.z
error: src->swizzle[i] < num_components (../src/compiler/nir/nir_validate.c:216)
```

the problem is attempting to reapply the component offset to a variable which is
already at an offset

fixes #10567

Fixes: 17a35412dc ("zink: re-rework i/o variable handling to make having variables entirely optional")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27522>
(cherry picked from commit 0a243a7241)

.pick_status.json

@@ -2414,7 +2414,7 @@
         "description": "zink: adjust swizzled deref loads by the variable component offset",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "17a35412dcc77a3058107100d27f734796a9d0ca",
         "notes": null

src/gallium/drivers/zink/zink_compiler.c

@@ -3764,7 +3764,7 @@ add_derefs_instr(nir_builder *b, nir_intrinsic_instr *intr, void *data)
          }
          /* filter needed components */
          if (intr->num_components < load->num_components)
-            load = nir_channels(b, load, BITFIELD_MASK(intr->num_components) << c);
+            load = nir_channels(b, load, BITFIELD_MASK(intr->num_components) << (c - var->data.location_frac));
          nir_def_rewrite_uses(&intr->def, load);
       } else {
          nir_def *store = intr->src[0].ssa;