[LLVM 21] Remove VectorizationFactor, use CreateElementCount.
LLVM 21 changes the IRBuilderBase::CreateVScale method to just do what
its name implies, to create a call to the @llvm.vscale intrinsic
function. In all instances where we were using it, we could have used
IRBuilder::CreateElementCount instead. To allow this to be used more
easily, this change removes compiler::utils::VectorizationFactor and
uses llvm::ElementCount instead, which does the same thing.