[llvm][SLP] Exit early if inputs to comparator are equal

**TL;DR:** This PR modifies a comparator. The comparator is used in a subsequent call to llvm::stable_sort. Sorting comparators should follow strict weak ordering - in particular, (x < x) should return false. This PR adds a fix to avoid an infinite loop when the inputs to the comparator are equal.

**Details**:

Sometimes when two equivalent tensors passed into the comparator, we encounter infinite looping (at https://github.com/llvm/llvm-project/blob/aae2eaae2cefd3132059925c4592276defdb1faa/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp#L4049)

Although it seems like this comparator will never be called with two equivalent pointers, some sanitizers, e.g. https://chromium.googlesource.com/chromiumos/third_party/gcc/+/refs/heads/stabilize-zako-5712.88.B/libstdc++-v3/include/bits/stl_algo.h#360, will add checks for (x < x). When this sanitizer is used with the current implementation, it triggers a comparator check for (x < x) which runs into the infinite loop

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D155874

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