[llvm] [RISCV] Use FLI + FNEG to materialize some negative FP constants (PR #70825)

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

LGTM, good spot that this can be improved. I think the way this patches modifies `getLegalZfaFPImm` makes sense, keeping logic changes minimal. But there's perhaps room for a follow-up refactoring that exploits knowledge about the table only having one negative constant and so avoids calling itself again.

https://github.com/llvm/llvm-project/pull/70825