nir: Rename lower_double_pack to lower_64bit_pack

There's nothing "double" about it other than, perhaps, the fact that it
packs two 32-bit values.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir.h

@@ -2549,7 +2549,7 @@ typedef enum {
 } nir_lower_doubles_options;
 
 void nir_lower_doubles(nir_shader *shader, nir_lower_doubles_options options);
-void nir_lower_double_pack(nir_shader *shader);
+void nir_lower_64bit_pack(nir_shader *shader);
 
 bool nir_normalize_cubemap_coords(nir_shader *shader);