Summary:
mark myindex as inline only and hide it from profilers.
Also prevent this function from being used any other way
than just for inlining.

Test Plan: ./validate

Reviewers: austin, erikd, simonmar, bgamari

Reviewed By: bgamari

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2715