[RISCV] Store fli min/nan/inf in index form in RISCVOperand.

Instead of converting to FP value, store them as Index using an
immediate operand.

Do the same for the explicit index form.

This avoids using the FP32 version of these special values as the
representation for fli.h and fli.d. inf/nan aren't so bad, but
"min" is problematic as the current implementation allows fli.d and
fli.h to accept the decimal version of the fp32 minimum value. I
will submit another patch to fix that.

Differential Revision: https://reviews.llvm.org/D145535