[VPlan] Use predicate from VPValue VPWidenSelectR::computeCost. (#172915)

Instead of looking up the predicate from the VPValue condition instead
of the underlying IR.

This improves cost modeling in some cases, e.g. when we can fold
operations like negations in compares. On AArch64, this leads to
additional vectorization in a few cases in practice.

Example lowering for the modified test case:
https://llvm.godbolt.org/z/6nc6jo5eG

PR: #172915

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -1964,9 +1964,8 @@ InstructionCost VPWidenSelectRecipe::computeCost(ElementCount VF,
   if (!ScalarCond)
     CondTy = VectorType::get(CondTy, VF);
 
-  CmpInst::Predicate Pred = CmpInst::BAD_ICMP_PREDICATE;
-  if (auto *Cmp = dyn_cast<CmpInst>(SI->getCondition()))
-    Pred = Cmp->getPredicate();
+  llvm::CmpPredicate Pred;
+  match(getOperand(0), m_Cmp(Pred, m_VPValue(), m_VPValue()));
   return Ctx.TTI.getCmpSelInstrCost(
       Instruction::Select, VectorTy, CondTy, Pred, Ctx.CostKind,
       {TTI::OK_AnyValue, TTI::OP_None}, {TTI::OK_AnyValue, TTI::OP_None}, SI);

llvm/test/Transforms/LoopVectorize/AArch64/select-costs.ll

@@ -92,10 +92,10 @@ exit:
 
 define i32 @select_xor_cond(ptr %src, i1 %c.0) {
 ; CHECK: LV: Checking a loop in 'select_xor_cond'
-; CHECK: Cost of 6 for VF 2: WIDEN-SELECT ir<%sel> = select ir<%c>, ir<false>, ir<%c.0>
-; CHECK: Cost of 12 for VF 4: WIDEN-SELECT ir<%sel> = select ir<%c>, ir<false>, ir<%c.0>
-; CHECK: Cost of 24 for VF 8: WIDEN-SELECT ir<%sel> = select ir<%c>, ir<false>, ir<%c.0>
-; CHECK: Cost of 48 for VF 16: WIDEN-SELECT ir<%sel> = select ir<%c>, ir<false>, ir<%c.0>
+; CHECK: Cost of 1 for VF 2: WIDEN-SELECT ir<%sel> = select ir<%c>, ir<false>, ir<%c.0>
+; CHECK: Cost of 1 for VF 4: WIDEN-SELECT ir<%sel> = select ir<%c>, ir<false>, ir<%c.0>
+; CHECK: Cost of 1 for VF 8: WIDEN-SELECT ir<%sel> = select ir<%c>, ir<false>, ir<%c.0>
+; CHECK: Cost of 1 for VF 16: WIDEN-SELECT ir<%sel> = select ir<%c>, ir<false>, ir<%c.0>
 ; CHECK: LV: Selecting VF: 4.
 
 entry: