[llvm] [CodeGen][MRI] Add allocation mask for register classes (PR #86007)

[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/86007