Improve generated code for typeassert with abstract type
The motivation is something like the following:
```
@noinline r_typeassert(c) = c ? (1,1) : nothing
function f_typeassert(c)
r(c)::Tuple
end
```
Here, we know that the return type from r_typeassert is either
`Tuple{Int, Int}` or `Nothing`, so all the type assert has to
do is assert that it's the former. We can compute this by
narrowing the type to be asserted using type intersection.
