Replace undefined with vector_init in vector code

[Timmmm]

There's a few places in the vector code like this:

val init_masked_result : forall 'n 'm 'p, 'n >= 0. (int('n), int('m), int('p), vector('n, dec, bits('m)), vector('n, dec, bool)) -> (vector('n, dec, bits('m)), vector('n, dec, bool))
function init_masked_result(num_elem, SEW, LMUL_pow, vd_val, vm_val) = {
...
  var mask : vector('n, dec, bool) = undefined;
  var result : vector('n, dec, bits('m)) = undefined;

I think that was done because until recently there was no way to create a vector literal without a fixed size. However this use of undefined seems pretty dubious. We should now be able to do something like this:

  var mask : vector('n, dec, bool) = vector_init(false);
  var result : vector('n, dec, bits('m)) = vector_init(zeros());

[Timmmm]

Some of that vector code looks like it could be rewritten in a nicer style without for loops anyway though.

[Timmmm]

I think the remaining undefineds are all covered by #554 (switching from vec(bool) to bits()) which I am working on (it's a decent sized change though).