[llvm] [SystemZ] Add DAGCombine for FCOPYSIGN to remove rounding. (PR #136131)

[Jonas Paulsson via llvm-commits]

JonPsson1 wrote:

Looking at CanCombineFCOPYSIGN_EXTEND_ROUND(), it seems that the reason the f128 case isn't handled in common code is that there is a specific guard against it because of x86. Maybe an option now would be to have a DAGCombine target hook that SystemZ would override, which would eventually be removed if x86 isel is improved per the comment. Or maybe just a comment there saying that the SystemZ handling could be removed if this changed?

https://github.com/llvm/llvm-project/pull/136131