[RISCV] Rename RISCVISD::SINT_TO_FP_VL/UINT_TO_FP_VL. NFC

Name them after the instructions VFCVT_RTZ_X(U)_F_VL to make it
clear that the ISD nodes don't have the poison semantics of
ISD::SINT_TO_FP/UINT_TO_FP.

I play to reuse this node for a FP_TO_SINT_SAT/FP_TO_UINT_SAT
patch and need the instruction semantics.