nir: Initialize evaluate_cube_face_index_amd dst.x.

Fix defect reported by Coverity Scan.

Uninitialized scalar variable (UNINIT)
uninit_use: Using uninitialized value dst.x.

Fixes: a1a2a8dfda ("nir: add AMD_gcn_shader extended instructions")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12290>

src/compiler/nir/nir_opcodes.py

@@ -537,6 +537,7 @@ dst.y = dst.y * (1.0f / ma) + 0.5f;
 """)
 
 unop_horiz("cube_face_index_amd", 1, tfloat32, 3, tfloat32, """
+dst.x = 0.0;
 float absX = fabsf(src0.x);
 float absY = fabsf(src0.y);
 float absZ = fabsf(src0.z);