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

[Craig Topper via llvm-commits]

topperc wrote:

I removed this in #108316. isLegalFPImm is not supposed to be in sync with lowerConstantFP. lowerConstantFP handles things that aren't legal.

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