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

[John Brawn via llvm-commits]

john-brawn-arm wrote:

#190838 means that the changes to do with comparing against the legacy cost model are no longer necessary, which simplifies this to just the change to using vplan for the cmp cost.

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