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Assertion failure when executing RISC-V vfncvt.rtz.x.f.w instruction
Description of problem:
When emulating the `vfncvt.rtz.x.f.w` instruction, QEMU crashes with an assertion failure at `target/riscv/translate.c:211`, complaining that ```decode_save_opc: Assertion `ctx->insn_start != NULL' failed.```
It appears this problem first emerged with https://gitlab.com/qemu-project/qemu/-/commit/a9814e3e08d2aacbd9018c36c77c2fb652537848
Steps to reproduce:
The following C program triggers the assertion failure when built a sufficiently recent version of the Clang cross compiler (in my case 15.0.6):
```
/* test.c */
#include <riscv_vector.h>
#define LEN 4
int main(int argc, char *argv[]) {
double in[LEN];
int out[LEN];
vfloat64m1_t vf = vle64_v_f64m1(in, LEN);
vint32mf2_t vi = vfncvt_rtz_x_f_w_i32mf2(vf, LEN);
vse32_v_i32mf2(out, vi, LEN);
return 0;
}
```
The above `test.c` can be compiled and run as follows:
```
clang -O3 -march=rv64gcv -static test.c
qemu-riscv64 -cpu "rv64,zba=true,zbb=true,zbc=true,zbs=true,v=true,vlen=512,elen=64,vext_spec=v1.0" a.out
qemu-riscv64: ../target/riscv/translate.c:211: decode_save_opc: Assertion `ctx->insn_start != NULL' failed.
Segmentation fault (core dumped)
```
Additional information:
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