[LV] NFCI: Do cost comparison on InstructionCost directly.
Instead of performing the isMoreProfitable() operation on
InstructionCost::CostTy the operation is performed on InstructionCost
directly, so that it can handle the case where one of the costs is
This patch also changes the CostTy to be int64_t, so that the type is
wide enough to deal with multiplications with e.g. unsigned MaxTripCount.
Reviewed By: dmgreen
Differential Revision: https://reviews.llvm.org/D105113