[SLP][NFC]Make isFirstInsertElement a weak strict ordering comparator.

To be used correctly in a sort-like function, isFirstInsertElement
function must follow weak strict ordering rule, i.e.
isFirstInsertElement(IE1, IE1) should return false.

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -6659,6 +6659,8 @@ static bool areTwoInsertFromSameBuildVector(InsertElementInst *VU,
 /// buildvector sequence.
 static bool isFirstInsertElement(const InsertElementInst *IE1,
                                  const InsertElementInst *IE2) {
+  if (IE1 == IE2)
+    return false;
   const auto *I1 = IE1;
   const auto *I2 = IE2;
   const InsertElementInst *PrevI1;