[llvm] Add EVT::changeVectorElementCount and MVT:changeVectorElementCount (PR #182266)

[Chaitanya Koparkar via llvm-commits]

ckoparkar wrote:

Yeah I'll add those as well, I've created #184779 for it.

While trying to find places to use these functions, I noticed a lot of code in DagCombiner is using `getVectorVT` with NumElements. What should we do about these cases? Is the plan to transition DagCombiner to ElementCount slowly?

https://github.com/llvm/llvm-project/pull/182266