[llvm] [SystemZ] Improve shouldCoalesce() for i128. (PR #74942)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

It seems to me we don't really need to care about the live range of the 128-bit register at all.  Regalloc will have to handle that no matter what, whether we coalesce or not.  Maybe we actually only need to check the live range of the 64-bit register itself - if there's 128-bit registers available during that range, then coalescing shouldn't make the situation any worse, and should be allowed.

https://github.com/llvm/llvm-project/pull/74942