[llvm] [LV] Compute register usage for interleaving on VPlan. (PR #126437)

[Sam Tebbs via llvm-commits]

https://github.com/SamTebbs33 approved this pull request.

Looks good to me, thank you! I've pushed my PR that removes the old calculateRegisterUsage function here: https://github.com/llvm/llvm-project/pull/132190

https://github.com/llvm/llvm-project/pull/126437