[clang] [llvm] [CIR] Use `llvm::ElementCount` to hold `cir::VectorType` size (PR #231906)

[Benjamin Maxwell via cfe-commits]

MacDue wrote:

If we're happy merging this soon, I think it'd be better to fix-up scalable vector issues one this API lands (as otherwise any fix-ups will need rework again).

https://github.com/llvm/llvm-project/pull/231906