[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 aae2eaae2c/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

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -4136,6 +4136,8 @@ BoUpSLP::getReorderingData(const TreeEntry &TE, bool TopToBottom) {
     return TE.ReorderIndices;
   if (TE.State == TreeEntry::Vectorize && TE.getOpcode() == Instruction::PHI) {
     auto PHICompare = [](llvm::Value *V1, llvm::Value *V2) {
+      if (V1 == V2)
+        return false;
       if (!V1->hasOneUse() || !V2->hasOneUse())
         return false;
       auto *FirstUserOfPhi1 = cast<Instruction>(*V1->user_begin());