use the right translation for inserted IT instructions

During mangling, and also for our proposed mechanism of drmgr automatically adding IT instrs in its final phase, we want the xl8of the added IT instrs to be the original app IT instr, if it existed, as using the first instr inside the block will not work on relocation. Should we insert labels to try to keep that info when we remove IT instrs? Should we mark IT instrs are non-safe-spots -- potentially resulting in too few safe spots (imagine a loop with an IT block body)?