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

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> that exploits knowledge about the table only having one negative constant and so avoids calling itself again.

I'm not sure if I understand you correctly, but right now we only recursively call `getLegalZfaFPImm` with negated FPImm if FPImm is negative. So I believe we've avoided most of the redundant recursive calls.

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