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

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

This should be a stacked review based of https://github.com/llvm/llvm-project/pull/126696 so that we can see how this patch changes the test.

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