[llvm] [Reassociate] Use uint64_t for repeat count (PR #94232)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I don't have a specific example, but there should probably be a comment with the invariants somewhere.   (If an operand can show up multiple times in the tree, you might run into issues?)

https://github.com/llvm/llvm-project/pull/94232