[LV] Widen ptr-inductions with scalar uses for scalable VFs.

Current codegen only supports scalarization of pointer inductions for
scalable VFs if they are uniform. After 3bebec65 we now may enter the
scalarization code path in VPWidenPointerInductionRecipe::execute for
scalable vectors.

Fall back to widening for scalable vectors if necessary.

This should fix a build failure when bootstrapping LLVM with SVE, e.g.
https://lab.llvm.org/buildbot/#/builders/176/builds/1723