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

[Vikash Gupta via llvm-commits]

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@@ -448,9 +448,21 @@ void MachineRegisterInfo::clearKillFlags(Register Reg) const {
 }
 
 bool MachineRegisterInfo::isLiveIn(Register Reg) const {
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vg0204 wrote:

 > I don't follow. We don't need any liveness tracking to check if a physical register is live into the function. Why can't you just s/MRI.isLiveIn(Reg)/MF.begin()->isLiveIn(Reg)/?

Because as the current MBB's isLiveIn defined it won't work as expected to get rid of the issue

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