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

Almost all recipes now go through ::computeCost to properly compute
their costs using the VPlan-based cost model. There are currently no
known cases where the VPlan-based cost model returns an incorrect cost
vs the legacy cost model. I check the remaining open issues with reports
of the assertion triggering and in all cases the VPlan-based cost model
is more accurate, which is causing the divergence.

There are still some fall-back paths, mostly via precomputeCosts, but
those cannot be easily removed without triggering the assert, as the
VPlan-based cost model is more accurate for those cases. An example of
this is https://github.com/llvm/llvm-project/pull/187056.

Fixes https://github.com/llvm/llvm-project/issues/38575. 
Fixes https://github.com/llvm/llvm-project/issues/149651. 
Fixes https://github.com/llvm/llvm-project/issues/182646. 
Fixes https://github.com/llvm/llvm-project/issues/183739. 
Fixes https://github.com/llvm/llvm-project/issues/187523.

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