Add a new unit attribute to allow for unsigned integer comparison.

Example:
```mlir
scf.for unsigned %iv_32 = %lb_32 to %ub_32 step %step_32 : i32 {
  // body
}
```

Discussion:
https://discourse.llvm.org/t/scf-should-scf-for-support-unsigned-comparison/84655

Change local variants to use new central one.

See https://github.com/llvm/llvm-project/pull/147168 for more info.

fix issue https://github.com/llvm/llvm-project/issues/146990

We can use *Set::insert_range to collapse:

  for (auto Elem : Range)
    Set.insert(E);

down to:

  Set.insert_range(Range);

In some cases, we can further fold that into the set declaration.

Fixes #130095.

When pipelining an `scf.for` with dynamic loop bounds, the epilogue
ramp-down must align with the prologue when num_stages >
total_iterations.

For example:
```
scf.for (0..ub) {
  load(i)
  add(i)
  store(i)
}
```
When num_stages=3 the pipeline follows:
```
load(0)  -  add(0)      -  scf.for (0..ub-2)    -  store(ub-2)
            load(1)     -                       -  add(ub-1)     -  store(ub-1)

```
The trailing `store(ub-2)`, `i=ub-2`, must align with the ramp-up for
`i=0` when `ub < num_stages-1`, so the index `i` should be `max(0,
ub-2)` and each subsequent index is an increment. The predicate must
also handle this scenario, so it becomes `predicate[0] =
total_iterations > epilogue_stage`.

This fixes loop iteration count calculation if the step is
    a negative value, where we should adjust the added
    delta from `step-1` to `step+1` when doing the ceil div.

The computed loop iteration is zero based, so only check it is less than
zero. This fixes the case when lower bound is not zero.