Don't use unsafeGlobalDynFlags in optCoercion

This plumbs DynFlags through CoreOpt so optCoercion can finally
eliminate its usage of `unsafeGlobalDynFlags`.

Note that this doesn't completely eliminate `unsafeGlobalDynFlags`
usage from this bit of the compiler. A few uses are introduced in
call-sites where we don't (yet) have ready access to `DynFlags`.

Test Plan: Validate

Reviewers: goldfire

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4774