[llvm] [VPlan] Don't use the legacy cost model for loop conditions (PR #156864)

[David Sherwood via llvm-commits]

https://github.com/david-arm commented:

I'm all in favour of making steps to retire the legacy cost model where possible! However, I think there are still a few issues that need dealing with particularly as the scope of this PR is actually a bit greater than it seems.

https://github.com/llvm/llvm-project/pull/156864