[NFC][LoopInterchange] Clean up of irrelevent dependency checking with

isOuterMostDepPositive()

The function isOuterMostDepPositive() is checked after negative dependence
vectors are normalized to be non-negative, so there will not be any negative
dependency ('>' as the outermost non-equal sign) after normalization. And
therefore the check in isOuterMostDepPositive() is irrelevent and redundant.

Reviewed By: congzhe

Differential Revision: https://reviews.llvm.org/D132982