[ConstantFolding] Use ICmpInst::Predicate instead of plain integer

The function `ConstantFoldCompareInstruction` uses `unsigned short` to
represent compare predicate, although all usesrs of the respective
include file use definition of CmpInst also. This change replaces
predicate argument type in this function to `ICmpInst::Predicate`,
which allows to make code a bit clearer and simpler.

No functional changes.

Differential Revision: https://reviews.llvm.org/D116379