[llvm] [SVE ACLE] Add `NodeMap[V]` for `svcount` in SelectionDAGBuilder (PR #126089)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

Is a test case necessary? It's an obvious non-breaking omission that I wouldn't have written a test case for if I had not originally omitted the code.

https://github.com/llvm/llvm-project/pull/126089