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

[Ulrich Weigand via llvm-commits]

uweigand wrote:

I think it might be conceptually simpler and also more general to -instead of adding this special-case heuristic- modify the base heuristics to always allow the GR128 live range to span multiple blocks, and only restrict the to-be-coalesced GR64 to only live within a single block.  (And still do the three-free-GR128 check in that latter block.)

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