[llvm] [LV][VPlan] Implement VPlan-based cost for exit condition. (PR #125640)

[Elvis Wang via llvm-commits]

ElvisWang123 wrote:

Gentle ping :)

The potential regression of `clamped_tc_8` is gone.  
If there are still some issues that this patch may cause regression, I will try to always add the cost of icmp to the VPBB (similar to cost of br) to match the legacy cost model.

https://github.com/llvm/llvm-project/pull/125640