[llvm] [llubi] Extract reusable methods of InstExecutor into ExecutorAPI (PR #186976)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

This looks reasonable enough to me at this point, but please wait for someone else to LGTM before landing.

https://github.com/llvm/llvm-project/pull/186976