RVV vfncvt.rtz.x.f.w Assertion failed
Description of problem:
when execute 
``` 
vsetvli        t0,       x0,         e16,      m1
vfncvt.rtz.x.f.w v0, v4
```
report error:

qemu-riscv64: ../target/riscv/translate.c:212: decode_save_opc: Assertion \`ctx->insn_start != NULL' failed. Segmentation fault (core dumped)
Steps to reproduce:
1. write the code
2. compile
3. excute
Additional information: