[llvm] [RA] Don't split a register generated from another split (PR #67351)

[via llvm-commits]

weiguozhi wrote:

> Looks good to me. Maybe add a short comment to explain why we bail out early in this case.

I copied the comment from splitAroundRegion which is for the same purpose.

https://github.com/llvm/llvm-project/pull/67351