[LV] Ignore candidate VFs with invalid costs.

This follows on from discussion on the mailing-list:
  https://lists.llvm.org/pipermail/llvm-dev/2021-June/151047.html

to interpret an Invalid cost as 'infinitely expensive', as this
simplifies some of the legalization issues with scalable vectors.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D105473