[VPlan] Only skip live-ins with constants in materializeBroadccast (NFC)

Currently this should be NFC, but will be needed in future patches.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -2203,8 +2203,8 @@ void VPlanTransforms::materializeBroadcasts(VPlan &Plan) {
   for (VPValue *VPV : VPValues) {
     if (all_of(VPV->users(),
                [VPV](VPUser *U) { return U->usesScalars(VPV); }) ||
-        (VPV->isLiveIn() &&
-         (!VPV->getLiveInIRValue() || isa<Constant>(VPV->getLiveInIRValue()))))
+        (VPV->isLiveIn() && VPV->getLiveInIRValue() &&
+         isa<Constant>(VPV->getLiveInIRValue())))
       continue;
 
     // Add explicit broadcast at the insert point that dominates all users.