[llvm] SystemZ: Implement copyPhysReg between vr128 and gr128 (PR #90616)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I am a little confused: If only 64-bits (one GPR) is defined and inserted, I would have thought a vector element load (VLVGG) would be used, but that would be something like a COPY from GR64 to VR64, except VR64 does not hold i64.

The point isn't to be optimal, the point is to ensure the liveness is tracked correctly 

> 
> Is it allowed to add an undef implicit use to the COPY source for the undef part..?
> 
> copy_gr128_to_vr128__r0q_to_v0_undef: COPY undef $r0q: maybe don't put r0q in the liveins list?

The point is the undef read on the def of the super register acts like a hidden IMPLICIT_DEF of the non-live half of the register 

https://github.com/llvm/llvm-project/pull/90616