[llvm] [LV] Remove legacy selectVectorizationFactor and assert (NFCI) (PR #190838)

[David Sherwood via llvm-commits]

david-arm wrote:

Looks like there is a test failure in LLVM.tools/UpdateTestChecks/update_analyze_test_checks/loopvectorize-costmodel.test. Once that's fixed I'm happy to accept the PR!

https://github.com/llvm/llvm-project/pull/190838