[LV] Use onlyFirstLaneDemanded when widening pointer phis (NFCI).

[fhahn]

Summary

This removes another instance of recipe execution still relying on
the cost model.

Depends on D116554.

[david-arm]

LGTM!

[fhahn]

Rebase on top of recent changes

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

case InductionDescriptor::IK_PtrInduction: {
 
     if (Cost->isScalarAfterVectorization(P, State.VF)) {
       // This is the normalized GEP that starts counting at zero.
       Value *PtrInd =
           Builder.CreateSExtOrTrunc(CanonicalIV, II.getStep()->getType());
       // Determine the number of scalars we need to generate for each unroll
       // iteration. If the instruction is uniform, we only need to generate the
       // first lane. Otherwise, we generate all VF values.
-      bool IsUniform = Cost->isUniformAfterVectorization(P, State.VF);
+      bool IsUniform = vputils::onlyFirstLaneUsed(PhiR);
       assert((IsUniform || !State.VF.isScalable()) &&
              "Cannot scalarize a scalable VF");
       unsigned Lanes = IsUniform ? 1 : State.VF.getFixedValue();
 
       for (unsigned Part = 0; Part < UF; ++Part) {
         Value *PartStart =
             createStepForVF(Builder, PtrInd->getType(), VF, Part);