spirv-fuzz: Wrap region in single iteration loop

This transformation should take region R and turn it into, essentially:

```
for (int new_counter = 0; new_counter < 1; new_counter++) {
R;
}
```

Several variations are possible here:

(1) The loop can be "for" style, with the check and break occurring in the loop head, or "do-while" style, with the check and break occurring at the back-edge block

(2) The loop counter can be a variable that is loaded and stored, or an id for which OpPhi captures the loop-carried dependence

(3) In the case where the loop counter is a variable, it can be Private or Function

(4) The counter could start from 0 and go to 1, or start from 1 and go to 0, and the check can use != or an ordered comparison.

(5) The counter could be integer or floating-point

Ideally the transformation should support all of these. I'm unsure whether this should be achieved by one monolithic transformation or a couple of different transformations. For instance, the "for" and "do-while" approach have different consequences for ids defined in the new loop and used after the loop, as the "for" approach changes dominance.

The issue names no file, test, or entry point. Start by surveying existing spirv-fuzz transformations and how they handle region R, then settle the loop form, counter representation, storage class, comparison, and numeric type before implementing. Done means the transformation supports the agreed variations and preserves the required dominance behavior.

Written by the indexing model from the issue text.