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

[Matt Arsenault via llvm-commits]

arsenm wrote:

FWIW I suspect we have other constraint bugs somewhere. In principle this patch is making the code more conservative. Does your problem go away if you add a getCommonSubClass check to the use and def rc? 

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