[llvm] [CodeGen] Allow negative frame indicies in Register class. (PR #164459)

[Mikhail Gudim via llvm-commits]

mgudim wrote:

> > I don't understand what a "negative frame index" is supposed to be. The `Register` class can encode references to stack slots which are indices into an array of stack objects in `MachineFrameInfo` and negative indexes make no sense there, no?
> 
> From MachineFrameInfo.h
> 
> ```
> /// To support this, the class assigns unique integer identifiers to stack       
> /// objects requested clients.  These identifiers are negative integers for      
> /// fixed stack objects (such as arguments passed on the stack) or nonnegative   
> /// for objects that may be reordered.  Instructions which refer to stack        
> /// objects use a special MO_FrameIndex operand to represent these frame         
> /// indexes.  
> ```

@MatzeB @topperc @arsenm 
We expect vast majority of the indices to be positive and a few negative ones. In theory we could "shift" the values so that we use the same 30 bits but only represent say 100 negative values and the rest are positive. I can do this too if people think its necessary. 

https://github.com/llvm/llvm-project/pull/164459