rs6000: Fix fake vec_promote overload

rs6000: Fix fake vec_promote overload

rs6000-overload.def defines one instance of vec_promote so that it can be
registered with the front end.  Actual expansion of the vec_promote overload
is done with special-case code in rs6000-c.c.  During another cleanup, I
observed that the fake instance has the wrong number of arguments.  Fix that.

2021-12-17  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-overload.def (__builtin_vec_promote): Add second
	argument.