use CFGMutation
use CFGMutation

Summary:
This simplifies the code a bit (and makes changes in later diffs easier), and improves runtime complexity a bit as it avoids `find_insn` calls.

This is a behavior-preserving transformation.

Reviewed By: adicatana

Differential Revision: D31129738

fbshipit-source-id: 3580f206fddca261f95120dced00452f6b03f775

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex