Replace instanceof checks with their respective predicates

[trevor-scheer]

This replaces all instanceof GraphQLXYZ checks with their respective predicate functions.

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.

[JakeDawkins]

Lol what even was this?

[trevor-scheer]

@shadaj this is way back in history but can you recall if this was intentional or possibly a mistake?
Re: 58f07c4#diff-508d6e2cd0e11941b1548a2ea48a909dR139

[shadaj]

Looks like a mistake! I really hope there's no situation in which double-checking a type is actually needed 😄

[JakeDawkins]

@shadaj Was removing the second check the mistake, or was that correct? Like this was originally

const isOptional = !(type instanceof GraphQLNonNull || type.ofType instanceof GraphQLNonNull);

was the ofType incorrectly removed, or was that fine to remove?

[shadaj]

It was fine to remove the extra check, I believe that ofType was eliminated with changes to the IR format so only the top level type needed to be checked.