[mypyc] Support var arg in CallC, replace new_dict_op (#8948)
Related: mypyc/mypyc#709, mypyc/mypyc#734
Summary:
* introduce variable arguments in CallC
* replace old new_dict_op (which relies on a specialized emit callback)
with two CallC ops: dict_new_op and dict_build_op, which handles create
an empty dict and a dict from k-v pairs.
* fix related IR tests, especially separate the testDel case into three
subcases which test list, dict and attributes respectively.
