[PSA] Swapping out `_or_null` with `_if_present`

[Mogball]

@bzcheeseman

I personally would prefer to have one way to do the thing this function does, i.e. check for existence and then do the dyn_cast

+1.

I also prefer the new name. if_present is a superset of nonnull IMO. It doesn’t make sense to keep an extra function around just as an alias.

@preames

I think the error here was in allowing dyn_cast_if_null to share an implementation with dyn_cast_if_present. dyn_cast_if_null is working on a pointer type

dyn_cast_if_present can also work on “pointer-like” types, i.e. PointerUnion. So I don’t think it makes sense to restrict it to just pointer types. Having two functions that do the same thing also creates friction for new users who aren’t “used” to a particular thing. Which one are they supposed to use? Is this buried somewhere in the style guide? Why are there two different functions anyways?