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

Split off #80269 as
suggested.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1402,9 +1402,9 @@ bool vputils::onlyFirstLaneUsed(const VPValue *Def) {
                 [Def](const VPUser *U) { return U->onlyFirstLaneUsed(Def); });
 }
 
-bool vputils::onlyFirstPartUsed(VPValue *Def) {
+bool vputils::onlyFirstPartUsed(const VPValue *Def) {
   return all_of(Def->users(),
-                [Def](VPUser *U) { return U->onlyFirstPartUsed(Def); });
+                [Def](const VPUser *U) { return U->onlyFirstPartUsed(Def); });
 }
 
 VPValue *vputils::getOrCreateVPValueForSCEVExpr(VPlan &Plan, const SCEV *Expr,

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3391,7 +3391,7 @@ namespace vputils {
 bool onlyFirstLaneUsed(const VPValue *Def);
 
 /// Returns true if only the first part of \p Def is used.
-bool onlyFirstPartUsed(VPValue *Def);
+bool onlyFirstPartUsed(const VPValue *Def);
 
 /// Get or create a VPValue that corresponds to the expansion of \p Expr. If \p
 /// Expr is a SCEVConstant or SCEVUnknown, return a VPValue wrapping the live-in