[llvm] [VPlan] First step towards VPlan cost modeling. (PR #92555)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

unreduced input, running through `opt -O3` should reproduce the crash: [a.ll.txt](https://github.com/user-attachments/files/15875049/a.ll.txt)

this was from building chromium ([instructions](https://www.chromium.org/developers/how-tos/get-the-code/)), specifically on mac x64, although I think I saw this on Linux x64 as well. `is_debug = false` is the important gn argument to set.

I'm also fine with recommitting this change with fixes and seeing if anything else breaks, or I can test a change for you by building a couple of configurations

https://github.com/llvm/llvm-project/pull/92555