Remove vector of bools in vext

[rez5427]

fix #554
emmm...
I will test the code with riscv-vector-tests later...
And to make sure assert('n > 0); is right

[github-actions]

Test Results

396 tests  ±0   396 ✅ ±0   0s ⏱️ ±0s
  4 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 819e0ac. ± Comparison against base commit 07fa23e.

♻️ This comment has been updated with latest results.

[rez5427]

The 'n produced in here, and I believe it can't be 0.

/* num_elem means max(VLMAX,VLEN/SEW)) according to Section 5.4 of RVV spec */
val get_num_elem : (int, int) -> nat
function get_num_elem(LMUL_pow, SEW) = {
  let LMUL_pow_reg = if LMUL_pow < 0 then 0 else LMUL_pow;
  /* Ignore lmul < 1 so that the entire vreg is read, allowing all masking to
   * be handled in init_masked_result */
  let num_elem = 2 ^ (LMUL_pow_reg) * VLEN / SEW;
  assert(num_elem > 0);
  num_elem
}

And the result of this commit run with riscv-vector-tests is in here