* add tests to reproduce #3177.
Any random test circuits can be added to t_split_var_4.v later because it uses CRC to check the result while
t_split_var_0.v has just barrel shifters.
* Fix#3177. Don't merge assign statements if a variable is marked split_var.
The intention was to not merge impure assignments, but the actual
predicate failed if the assignment was indeed pure.
This fix gains 1.5% speed on SweRV EH1.
* test:Add more tests for checking split_var for unpacked array.
* Fix range calculation of SliceSel and change to UASSERT_OBJ because the range check is done in V3Width beforehand.
