[llvm] [CodeGen][MRI] Introduce synthetic register classes (PR #86006)

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

Posted RFC https://discourse.llvm.org/t/rfc-synthetic-register-classes-and-allocation-mask-to-determine-the-final-allocation-order/77853

https://github.com/llvm/llvm-project/pull/86006