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

[Xinlong Chen via llvm-commits]

Xinlong-Chen wrote:

> 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

Good idea~ I have split it into two commits, and the codegen diff from the new functionality is at: https://github.com/llvm/llvm-project/pull/193672/files/0e037f0b503b3562295823714d26be760be80ea3

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