[llvm] [LV] Cache call vectorization decisions (PR #66521)

[via llvm-commits]

huntergr-arm wrote:

Moved this to github to get used to the new workflow.
I believe I've taken care of David's requests for the code, let me know if I missed something.

The changes to using .at() instead of operator[] were because I was performing lookups in a const function, and operator[] is not const (it can create new entries). .at() will assert if the entry does not exist.

I would have split off the other part involving the unit test change, but it appears that the cost was printed out twice and another commit has already updated the test so it no longer fails with my changes.

https://github.com/llvm/llvm-project/pull/66521