[llvm] [CodeGen] Utilizing register units based liveIns tracking in MBB (PR #129847)

[Vikash Gupta via llvm-commits]

vg0204 wrote:

Also, the liveIn registers represent physReg which need not be entirely live into MBB (so the laneMask makes sense), but migrating entirely into regUnits (representing only live parts of liveIn physRegs) could cause severe fallout as you mentioned at use sites. So, we strongly cannot eliminate the register concept as liveIn of MBB I fee!

Could you through some light on it @arsenm!

https://github.com/llvm/llvm-project/pull/129847