Audit usage of `var` and `undefined`

[Timmmm]

There are many uses of var and undefined that are unnecessary and dodgy, especially in the vector code. For example this:

  var result  : vector('n, dec, bits('o)) = undefined;
  var mask    : vector('n, dec, bool)     = undefined;

  (result, mask) = init_masked_result(num_elem, SEW_widen, LMUL_pow_widen, vd_val, vm_val);

  foreach (i from 0 to (num_elem - 1)) {
    if mask[i] then {
      result[i] = match funct6 {
        FWVV_VMACC   => fp_muladd(rm_3b, fp_widen(vs1_val[i]), fp_widen(vs2_val[i]), vd_val[i]),
        FWVV_VNMACC  => fp_nmulsub(rm_3b, fp_widen(vs1_val[i]), fp_widen(vs2_val[i]), vd_val[i]),
        FWVV_VMSAC   => fp_mulsub(rm_3b, fp_widen(vs1_val[i]), fp_widen(vs2_val[i]), vd_val[i]),
        FWVV_VNMSAC  => fp_nmuladd(rm_3b, fp_widen(vs1_val[i]), fp_widen(vs2_val[i]), vd_val[i])
      }
    }
  };
  write_vreg(num_elem, SEW_widen, LMUL_pow_widen, vd, result);

Would probably be better like

  let (result_, mask) = init_masked_result(num_elem, SEW_widen, LMUL_pow_widen, vd_val, vm_val);
  var result = result_;

  foreach (i from 0 to (num_elem - 1)) {
    if mask[i] then {
      result[i] = match funct6 {
        FWVV_VMACC   => fp_muladd(rm_3b, fp_widen(vs1_val[i]), fp_widen(vs2_val[i]), vd_val[i]),
        FWVV_VNMACC  => fp_nmulsub(rm_3b, fp_widen(vs1_val[i]), fp_widen(vs2_val[i]), vd_val[i]),
        FWVV_VMSAC   => fp_mulsub(rm_3b, fp_widen(vs1_val[i]), fp_widen(vs2_val[i]), vd_val[i]),
        FWVV_VNMSAC  => fp_nmuladd(rm_3b, fp_widen(vs1_val[i]), fp_widen(vs2_val[i]), vd_val[i])
      }
    }
  };

  write_vreg(num_elem, SEW_widen, LMUL_pow_widen, vd, result);

(unfortunately var result = result; doesn't work)