[SLP]Avoid multiple attempts to vectorize CmpInsts.

No need to lookup through and/or try to vectorize operands of the
CmpInst instructions during attempts to find/vectorize min/max
reductions. Compiler implements postanalysis of the CmpInsts so we can
skip extra attempts in tryToVectorizeHorReductionOrInstOperands and save
compile time.

Differential Revision: https://reviews.llvm.org/D99950