[VPlan] Use predicate in VPInstruction::computeCost for selects.

In some cases, the lowering a select depends on the predicate. If the
condition of a select is a compare instruction, thread the predicate
through to the TTI hook.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -1008,9 +1008,8 @@ InstructionCost VPInstruction::computeCost(ElementCount VF,
 
   switch (getOpcode()) {
   case Instruction::Select: {
-    // TODO: It may be possible to improve this by analyzing where the
-    // condition operand comes from.
-    CmpInst::Predicate Pred = CmpInst::BAD_ICMP_PREDICATE;
+    llvm::CmpPredicate Pred = CmpInst::BAD_ICMP_PREDICATE;
+    match(getOperand(0), m_Cmp(Pred, m_VPValue(), m_VPValue()));
     auto *CondTy = Ctx.Types.inferScalarType(getOperand(0));
     auto *VecTy = Ctx.Types.inferScalarType(getOperand(1));
     if (!vputils::onlyFirstLaneUsed(this)) {

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

@@ -76,9 +76,9 @@ exit:
 
 define i32 @select_vpinst_for_tail_folding(i8 %n) {
 ; CHECK: LV: Checking a loop in 'select_vpinst_for_tail_folding'
-; CHECK: Cost of 6 for VF 2: EMIT vp<{{.+}}> = select vp<{{.+}}>, ir<%red.next>, ir<%red>
-; CHECK: Cost of 12 for VF 4: EMIT vp<{{.+}}> = select vp<{{.+}}>, ir<%red.next>, ir<%red>
-; CHECK: LV: Selecting VF: 1
+; CHECK: Cost of 1 for VF 2: EMIT vp<{{.+}}> = select vp<{{.+}}>, ir<%red.next>, ir<%red>
+; CHECK: Cost of 1 for VF 4: EMIT vp<{{.+}}> = select vp<{{.+}}>, ir<%red.next>, ir<%red>
+; CHECK: LV: Selecting VF: 4
 
 entry:
   %c = icmp ne i8 %n, 0