[LoopInfo] Fix function getInductionVariable

The way function gets the induction variable is by judging whether
StepInst or IndVar in the phi statement is one of the operands of CMP.
But if the LatchCmpOp0/LatchCmpOp1 is a constant,  the subsequent
comparison may result in null == null, which is meaningless. This patch
fixes the typo.

Reviewed By: Whitney

Differential Revision: https://reviews.llvm.org/D112980