[mypyc] Coerce Register assignments (#11176)
Fixes mypyc/mypyc#759 and probably some other issues. Including only
the change to coerce the Register assignments fixes the missing
type coercion for the specialized next-generator case, but causes some
unusual box-then-unbox from short_ints.
These odd coercions are caused by the dict keys in dict literals being
considered short_int, but dict_next_rtuple_single/pair return types bein
annotated as int_primitive. Changing this to short_int_rprimitive removes
the odd coercisions. The remainder of the diff is fixing the irbuild
testcase output to have short_int.
