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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> SILowerSGPRSpill, where KILL flag is introduced based on MRI live-in list.

Can this look at the block live ins instead? 

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