Class InstrProfIncrementInstStep inherits from InstrProfIncrementInst but cannot cast to InstrProfIncrementInst, because InstrProfIncrementInst::classof does not cover such circumstance。
Function InstrProfiling::run traverse all instruction in a module and try to cast them to InstrProfIncrementInst using dyn_cast, but it will return nullptr if the instruction is InstrProfIncrementInstStep(subclass of InstrProfIncrementInst).
Reviewed By: tejohnson, ellis
Differential Revision: https://reviews.llvm.org/D141579
259fa70885