[SLP]Fix PR79321: SLPVectorizer's PHICompare doesn't provide a strict

weak ordering.

Compared NumUses to meet the reaquirements of the strict weak ordering.

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -4138,9 +4138,11 @@ BoUpSLP::getReorderingData(const TreeEntry &TE, bool TopToBottom) {
     auto PHICompare = [&](unsigned I1, unsigned I2) {
       Value *V1 = TE.Scalars[I1];
       Value *V2 = TE.Scalars[I2];
-      if (V1 == V2)
+      if (V1 == V2 || (V1->getNumUses() == 0 && V2->getNumUses() == 0))
         return false;
-      if (!V1->hasOneUse() || !V2->hasOneUse())
+      if (V1->getNumUses() < V2->getNumUses())
+        return true;
+      if (V1->getNumUses() > V2->getNumUses())
         return false;
       auto *FirstUserOfPhi1 = cast<Instruction>(*V1->user_begin());
       auto *FirstUserOfPhi2 = cast<Instruction>(*V2->user_begin());

llvm/test/Transforms/SLPVectorizer/X86/extract-subvector-long-input.ll

@@ -7,7 +7,7 @@ define void @test() {
 ; CHECK-NEXT:    br label [[BB1:%.*]]
 ; CHECK:       bb1:
 ; CHECK-NEXT:    [[TMP0:%.*]] = phi <8 x i32> [ poison, [[BB10:%.*]] ], [ zeroinitializer, [[BB:%.*]] ]
-; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <8 x i32> [[TMP0]], <8 x i32> poison, <2 x i32> <i32 6, i32 6>
+; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <8 x i32> [[TMP0]], <8 x i32> poison, <2 x i32> <i32 7, i32 7>
 ; CHECK-NEXT:    switch i32 0, label [[BB16:%.*]] [
 ; CHECK-NEXT:      i32 0, label [[BB14:%.*]]
 ; CHECK-NEXT:      i32 1, label [[BB11:%.*]]