[llvm] [AMDGPU][MachineRegisterInfo] Extend the MRI live-ins check to account for Subreg (PR #126926)

[Vikash Gupta via llvm-commits]

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@@ -1020,7 +1020,7 @@ class MachineRegisterInfo {
     return LiveIns;
   }
 
-  bool isLiveIn(Register Reg) const;
+  bool isLiveIn(Register Reg, bool CheckForSubreg = false) const;
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vg0204 wrote:

> However, I still think we should replace all of the livein references to be directly in terms of regunits

Makes sense to me, but it would quite a huge task, as live-ins appeared & represented differently across different places need to be handled :

- In MBB, live-ins are stored as physreg + lane mask (regunit based implementation)
- In MachinFunction, live-ins are just pair of <PhysReg, VirtualReg> in association
- In MachineVerfier, MBB's live-ins are calculated as the physReg & all its subregs  

https://github.com/llvm/llvm-project/pull/126926