Fix off-by-one in generator nospecialize (#50571)
I had an off-by-one in #50556, since the argument slots actually start
at 2 and `iota` starts at `0`. This was breaking StaticArrays
precompiles, which attempts to precompile a generator with its abstract
signature and without the nospecialize, those signatures are not
compileable.
Co-authored-by: Oscar Smith <oscardssmith@gmail.com>
