[llvm] [TableGen][GISel] Learn to import patterns with physreg defs (PR #120343)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> You can find the smallest allocatable class with with getAllocatableClass(getMinimalPhysRegClass()), and there's presumably a CodeGenRegisters equivalent too
> 

I think this is iterate through getRegisterClass(RegRecord)->getSuperClasses() until you find an allocatable one 

https://github.com/llvm/llvm-project/pull/120343