[VPlan] Mark BranchOnCount and BranchOnCond as having side effects (NFC)

BranchOnCount and BranchOnCond do not read memory, but cannot be moved.

Mark them as having side-effects, but not reading/writing memory, which
more accurately models that above. This allows removing some special
checking for branches both in the current code and future patches.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -162,8 +162,12 @@ bool VPRecipeBase::mayHaveSideEffects() const {
   case VPPredInstPHISC:
   case VPVectorEndPointerSC:
     return false;
-  case VPInstructionSC:
-    return mayWriteToMemory();
+  case VPInstructionSC: {
+    auto *VPI = cast<VPInstruction>(this);
+    return mayWriteToMemory() ||
+           VPI->getOpcode() == VPInstruction::BranchOnCount ||
+           VPI->getOpcode() == VPInstruction::BranchOnCond;
+  }
   case VPWidenCallSC: {
     Function *Fn = cast<VPWidenCallRecipe>(this)->getCalledScalarFunction();
     return mayWriteToMemory() || !Fn->doesNotThrow() || !Fn->willReturn();
@@ -1241,6 +1245,8 @@ bool VPInstruction::opcodeMayReadOrWriteFromMemory() const {
   case Instruction::Select:
   case Instruction::PHI:
   case VPInstruction::AnyOf:
+  case VPInstruction::BranchOnCond:
+  case VPInstruction::BranchOnCount:
   case VPInstruction::Broadcast:
   case VPInstruction::BuildStructVector:
   case VPInstruction::BuildVector:

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -4174,7 +4174,7 @@ void VPlanTransforms::narrowInterleaveGroups(VPlan &Plan, ElementCount VF,
   unsigned VFMinVal = VF.getKnownMinValue();
   SmallVector<VPInterleaveRecipe *> StoreGroups;
   for (auto &R : *VectorLoop->getEntryBasicBlock()) {
-    if (isa<VPCanonicalIVPHIRecipe>(&R) || match(&R, m_BranchOnCount()))
+    if (isa<VPCanonicalIVPHIRecipe>(&R))
       continue;
 
     if (isa<VPDerivedIVRecipe, VPScalarIVStepsRecipe>(&R) &&