[llvm] [LoopVectorize] Teach some X86 cost model tests to use new vplan costs (PR #120738)

[David Sherwood via llvm-commits]

david-arm wrote:

> Thanks for cleaning this up. Might be good to document the recommended command to semi-automatically update the tests.

Heh, the script I used to do this is probably quite embarrassing! But yeah I agree with you - if I can come up with a nice way of documenting I'll put it somewhere.

https://github.com/llvm/llvm-project/pull/120738