[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:

> I think that if we do this, it should not be an option. It should simply be unconditional.
 
Need to see its side-effect on other architectures, as can't comment on their interpretation right! 

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