[ARM][CostModel] Improve getCastInstrCost

- Specifically check for sext/zext users which have 'long' form NEON
  instructions.
- Add more entries to the table for sext/zexts so that we can report
  more accurately the number of vmovls required for NEON.
- Pass the instruction to the pass implementation.

Differential Revision: https://reviews.llvm.org/D79561