[VPlan] Mark vputils::onlyFirstLaneUsed arg as const (NFC)

Split off #80269 as suggested.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1397,9 +1397,9 @@ void VPSlotTracker::assignSlots(const VPBasicBlock *VPBB) {
       assignSlot(Def);
 }
 
-bool vputils::onlyFirstLaneUsed(VPValue *Def) {
+bool vputils::onlyFirstLaneUsed(const VPValue *Def) {
   return all_of(Def->users(),
-                [Def](VPUser *U) { return U->onlyFirstLaneUsed(Def); });
+                [Def](const VPUser *U) { return U->onlyFirstLaneUsed(Def); });
 }
 
 bool vputils::onlyFirstPartUsed(VPValue *Def) {

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3388,7 +3388,7 @@ class VPlanSlp {
 namespace vputils {
 
 /// Returns true if only the first lane of \p Def is used.
-bool onlyFirstLaneUsed(VPValue *Def);
+bool onlyFirstLaneUsed(const VPValue *Def);
 
 /// Returns true if only the first part of \p Def is used.
 bool onlyFirstPartUsed(VPValue *Def);