[PGO] incorrect classof in InstrProfIncrementInst

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