[llvm] [RISCV] Ignore debug instructions in RISCVVLOptimizer (PR #159616)

[Harald van Dijk via llvm-commits]

https://github.com/hvdijk approved this pull request.

This looks right, thanks, no debug instruction should ever require any particular VL because it's not a real instruction.

https://github.com/llvm/llvm-project/pull/159616