[llvm] [LV] Don't trigger legacy/vplan assert when forcing costs (PR #156870)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

Makes sense to me

In the PR description:

> the legacy cost model will treat an entire load interleave group as being a cost of 1, whereas the legacy cost model will treat each load in the group as having a cost of 1. 

Should this say the VPlan cost model will treat an entire load interleave group as being a cost of 1?

https://github.com/llvm/llvm-project/pull/156870