Allow trace to assign in two steps

With field sensitivity on arrays, the element can be initialized in two
steps, first the field f then the field array.

regression/cbmc/trace-values/trace-values.desc

@@ -11,7 +11,8 @@ trace-values.c
 ^  my_nested\[1.*\].f=5 .*$
 ^  junk\$object=7 .*$
 ^  dynamic_object1\[1.*\]=8 .*$
-^  my_nested\[1.*\]=\{ .f=0, .array=\{ 0, 4, 0 \} \} .*$
+^  my_nested\[1.*\](=\{ )?.f=0[ ,]
+^  my_nested\[1.*\](=\{ .f=0, )?.array=\{ 0, 4, 0 \}
 ^VERIFICATION FAILED$
 --
 ^warning: ignoring