use cfg_mutation.insert_(before/after)
use cfg_mutation.insert_(before/after)

Summary:
As in the title. Instead of inserting directly in the cfg via `find_insn`, enhance the
use the new CFGMutation API to insert positions before and after instructions.

Reviewed By: NTillmann

Differential Revision: D27396172

fbshipit-source-id: 0716a240041072b0ef90f150021c48f2a676e442

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