Use vmv_s_x instead of the constant will be materialized in a GPR.
This avoids going from GPR to FPR to vector.
We already did this for RISCVISD::VFMV_S_F_VL and probably we should
just turn int_riscv_vfmv_s_f into RISCVISD::VFMV_S_F_VL, but I'd like
to see some improvements to RISCVInsertVSETVLI first.