[LV] Use legacy cost model by default in 19.x

[fhahn]

As discussed in #92555, use the legacy cost model by default in the 19.x release using the option added in #99536.

[fhahn]

/cherry-pick a72a0bf

[fhahn]

cc @ayalz @nikic

[llvmbot]

Failed to cherry-pick: a72a0bf

https://github.com/llvm/llvm-project/actions/runs/10057148517

Please manually backport the fix and push it to your github fork. Once this is done, please create a pull request

[nikic]

Closing in favor of the PR #100097.