push struct and multi-packed ternaries into ranges

- the ternary expressions were previously used to decide the direction
  of generated part selects
- this fixes an issue where ternaries were generated for LHSs

test/basic/struct_part_select_param.v

@@ -0,0 +1,3 @@
+// The reference output for this test should match that of the
+// struct_part_select test.
+`include "struct_part_select.v"