[LoopVectorize] Classify pointer induction updates as scalar only if they have one use

collectLoopScalars collects pointer induction updates in ScalarPtrs, assuming
that the instruction will be scalar after vectorization. This may crash later
in VPReplicateRecipe::execute() if there there is another user of the instruction
other than the Phi node which needs to be widened.

This changes collectLoopScalars so that if there are any other users of
Update other than a Phi node, it is not added to ScalarPtrs.

Reviewed By: david-arm, fhahn

Differential Revision: https://reviews.llvm.org/D111294