[llvm] [AArch64][GlobalISel] Improve and expand fcopysign lowering (PR #71283)

[David Green via llvm-commits]

davemgreen wrote:

Thanks for the reviews and sorry for the delay on this one. There is now a separate optimization to produce fneg(movi 0), which should cover the places where this was previously potentially performing worse.

https://github.com/llvm/llvm-project/pull/71283