[RISCV] Rename RISCVISD::FMINNUM_VL/FMAXNUM_VL to VFMIN_VL/VFMAX_VL. NFC

I want these to have RISC-V semantics not LLVM IR semantics. Specifically
that -0.0 comes before +0.0.

This is needed to emulate FMAXIMUM/FMINIMUM for vectors.