Automerge: [SLP]Drop samesign flag if the vector node has reduced bitwidth

If the operands of the icmp instructions has reduced bitwidth after
MinBitwidth analysis, need to drop samesign flag to preserve correctness
of the transformation.

Fixes #120823

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -15483,6 +15483,8 @@ Value *BoUpSLP::vectorizeTree(TreeEntry *E, bool PostponedPHIs) {
       CmpInst::Predicate P0 = cast<CmpInst>(VL0)->getPredicate();
       Value *V = Builder.CreateCmp(P0, L, R);
       propagateIRFlags(V, E->Scalars, VL0);
+      if (auto *ICmp = dyn_cast<ICmpInst>(V); ICmp && It == MinBWs.end())
+        ICmp->setSameSign(/*B=*/false);
       // Do not cast for cmps.
       VecTy = cast<FixedVectorType>(V->getType());
       V = FinalShuffle(V, E);

llvm/test/Transforms/SLPVectorizer/X86/buildvector-schedule-for-subvector.ll

@@ -6,7 +6,7 @@ define void @test() {
 ; CHECK-NEXT:  [[BB:.*:]]
 ; CHECK-NEXT:    [[ADD:%.*]] = add i32 1, 0
 ; CHECK-NEXT:    [[TMP0:%.*]] = insertelement <4 x i32> <i32 0, i32 0, i32 0, i32 poison>, i32 [[ADD]], i32 3
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp samesign ult <4 x i32> [[TMP0]], zeroinitializer
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult <4 x i32> [[TMP0]], zeroinitializer
 ; CHECK-NEXT:    [[ICMP:%.*]] = extractelement <4 x i1> [[TMP1]], i32 2
 ; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[ICMP]], i32 0, i32 0
 ; CHECK-NEXT:    [[ZEXT:%.*]] = zext i32 [[SELECT]] to i64