[DAG] Add MVT:changeVectorElementCount and EVT::changeVectorElementCount

Similar to EVT::changeVectorElementType - but this will allow us to adjust the `ElementCount` on the fly.

We should assert that the original type was a vector already, but we will need to support Simple<->Extended transitions similar to EVT::changeVectorElementType.

There are a number of EVT::getVectorVT() calls in DAGCombine that would benefit from converting to changeVectorElementCount for clarity, but don't attempt to do too many in an initial patch