There are several places where we use `llvm::OwningArrayRef`. The
interface to this requires us to first construct temporary storage, then
allocate space and set the allocated memory to 0, then copy the values
we actually want into that memory, then move the array into place.
Instead we can just do it all inline in a single pass by using
`std::vector`. In one case we actually allocate a completely separate
container and then allocate + copy the data over because
`llvm::OwningArrayRef` does not (and can't) support `push_back`.

Note that `llvm::SmallVector` is not a suitable replacement here because
we rely on reference stability on move construction: when the outer
container reallocates, we need the the contents of the inner containers
to be fixed in memory, and `llvm::SmallVector` does not give us that
guarantee.

Use wrappers around `std::accumulate` to make the code more concise and
less bug-prone: https://github.com/llvm/llvm-project/pull/162129.

With `std::accumulate`, it's the initial value that determines the
accumulator type. `llvm::sum_of` and `llvm::product_of` pick the right
accumulator type based on the range element type.

Found some funny bugs like a local accumulate helper that calculated a
sum with initial value of 1 -- we didn't hit the bug because the code
was actually dead...

Use `DefaultUnreachable` from
https://github.com/llvm/llvm-project/pull/161970.

Skipping only over the first results leads to the curCodeIt pointing to
the wrong location in the bytecode, causing the execution to continue
with a wrong instruction after the Constraint/Rewrite.

Signed-off-by: Rickert, Jonas <Jonas.Rickert@amd.com>

I observed that we have the boundary comments in the codebase like:

```
//===----------------------------------------------------------------------===//
// ...
//===----------------------------------------------------------------------===//
```

I also observed that there are incomplete boundary comments. The
revision is generated by a script that completes the boundary comments.

```
//===----------------------------------------------------------------------===//
// ...

...
```

Signed-off-by: hanhanW <hanhan0912@gmail.com>