[llvm] SplitKit: Take register class directly from instruction definition (PR #129727)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

I've verified that this patch solves downstream problems I bisected to
https://github.com/llvm/llvm-project/pull/122110
which I mentioned in
https://github.com/llvm/llvm-project/pull/122110#issuecomment-2696651713

https://github.com/llvm/llvm-project/pull/129727