transform: Don't delete MIEs when deleting opcodes
transform: Don't delete MIEs when deleting opcodes

Summary:
Deleting MIEs gets us into sticky situations by invalidating
iterators. In particular, it's tricky to keep the CFG iterators in sync, and
can lead to segfaults. Instead we convert the MIEs into MFLOW_FALLTHROUGHs and
have them dropped when we sync().

Reviewed By: bertmaher

Differential Revision: D3936183

fbshipit-source-id: 3e36a8d3dd711911a2f4f7901e7727c10d183622

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