[llvm] [DAG] computeKnownFPClass - add ISD::FCOPYSIGN handling (PR #193672)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Add the new tests (with baseline codegen) as an initial commit in this PR, then the new functionality and test codegen changes in a second commit - makes it easier to demonstrate the code is working

https://github.com/llvm/llvm-project/pull/193672