Vector: Remove some unused result variables

There are some vector instructions that use `init_masked_result_cmp`
but do not use the returned result variable, so just discard it rather
than create a new variable.

model/riscv_insts_vext_mask.sail

@@ -98,8 +98,7 @@ function clause execute(VCPOP_M(vm, vs2, rd)) = {
  let vm_val : vector('n, dec, bool) = read_vmask(num_elem, vm, 0b00000);
  let vs2_val : vector('n, dec, bool) = read_vmask(num_elem, 0b0, vs2);
 
- let (initial_result, mask) = init_masked_result_cmp(num_elem, SEW, 0, vs2_val, vm_val);
- var result = initial_result;
+ let (_, mask) = init_masked_result_cmp(num_elem, SEW, 0, vs2_val, vm_val);
 
  var count : nat = 0;
  foreach (i from 0 to (num_elem - 1)) {
@@ -133,8 +132,7 @@ function clause execute(VFIRST_M(vm, vs2, rd)) = {
  let vm_val : vector('n, dec, bool) = read_vmask(num_elem, vm, 0b00000);
  let vs2_val : vector('n, dec, bool) = read_vmask(num_elem, 0b0, vs2);
 
- let (initial_result, mask) = init_masked_result_cmp(num_elem, SEW, 0, vs2_val, vm_val);
- var result = initial_result;
+ let (_, mask) = init_masked_result_cmp(num_elem, SEW, 0, vs2_val, vm_val);
 
  var index : int = -1;
  foreach (i from 0 to (num_elem - 1)) {