[LV] Keep track of cost-based ScalarAfterVec in VPWidenPointerInd.

Epilogue vectorization uses isScalarAfterVectorization to check if
widened versions for inductions need to be generated and bails out in
those cases.

At the moment, there are scenarios where isScalarAfterVectorization
returns true but VPWidenPointerInduction::onlyScalarsGenerated would
return false, causing widening.

This can lead to widened phis with incorrect start values being created
in the epilogue vector body.

This patch addresses the issue by storing the cost-model decision in
VPWidenPointerInductionRecipe and restoring the behavior before 151c1443.
This effectively reverts 151c1443, but the long-term fix is to properly
support widened inductions during epilogue vectorization

Fixes #57712.