[mypy] update check.inst to not blank out types

Summary:
inst and opt_inst were set-up to -> Any which meant that they would blank out a lot of typing information as things went through it.

Idealy the type argument to check could be uesd, but I can not figure out how, so instead just:

• for inst make sure existing type info flows through
• overload opt_inst to try to get that set up right too

Test Plan: mypy

Reviewers: dgibson, max, cdecarolis, owen

Reviewed By: cdecarolis

Differential Revision: https://dagster.phacility.com/D8898