[llvm] [WIP][CodeGen] Modifying MBB's liveins representation as into regUnits (PR #129847)

[Vikash Gupta via llvm-commits]

vg0204 wrote:

@jayfoad , should the issue #138552 need to be resolved completely before this works can be taken forward or can we check for regunit with laneBitmask as 0x0 to understand its a fake (or shared regunit) & setting all its bit as suggested by you in https://github.com/llvm/llvm-project/issues/78942

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