[llvm] [RISCV] Teach RISCVTargetLowering::isFPImmLegal about fli+fneg (PR #149075)

[Alex Bradbury via llvm-commits]

asb wrote:

Thanks for the reference. How would you define "legal" constants because by my reading of the function, it's currently counting anything that can be handled with two instructions (e.g. -0.0 via fmv x0 then fneg, and FP values with bit patterns that can be loaded with a single int instruction then fmv).

https://github.com/llvm/llvm-project/pull/149075