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

[Ulrich Weigand via llvm-commits]

uweigand wrote:

Given https://github.com/llvm/llvm-project/pull/90861/commits/abbd73e94c8512cb5bcd3d5eab538f55e7cb7d95, should we similarly just remove the subreg tests as redundant here?

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