Refactoring llvm-flo.cpp into a new class RewriteInstance, NFC.

Summary:
Previously, llvm-flo.cpp contained a long function doing lots of
different tasks. This patch refactors this logic into a separate class with
different member functions, exposing the relationship between each step of
the rewritting process and making it easier to coordinate/change it.

(cherry picked from FBD2691674)