rs6000: Fix the check of bif argument number [PR104482]

As PR104482 shown, it's one regression about the handlings when
the argument number is more than the one of built-in function
prototype.  The new bif support only catches the case that the
argument number is less than the one of function prototype, but
it misses the case that the argument number is more than the one
of function prototype.  Because it uses "n != expected_args",
n is updated in

   for (n = 0; !VOID_TYPE_P (TREE_VALUE (fnargs)) && n < nargs;
        fnargs = TREE_CHAIN (fnargs), n++)

, it's restricted to be less than or equal to expected_args with
the guard !VOID_TYPE_P (TREE_VALUE (fnargs)), so it's wrong.

The fix is to use nargs instead, also move the checking hunk's
location ahead to avoid useless further scanning when the counts
mismatch.

	PR target/104482

gcc/ChangeLog:

	* config/rs6000/rs6000-c.cc (altivec_resolve_overloaded_builtin): Fix
	the equality check for argument number, and move this hunk ahead.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr104482.c: New test.