[mlir] [EDSC] Add interface for yield-for loops.

[mlir] [EDSC] Add interface for yield-for loops.

Summary:
ModelBuilder was missing an api to easily generate yield-for-loops.
This diffs implements an interface allowing to write:
```
%2:2 = loop.for %i = %start to %end step %step iter_args(%arg0 = %init0, %arg1 = %init1) -> (f32, f32) {
  %sum = addf %arg0, %arg1 : f32
  loop.yield %arg1, %sum : f32, f32
}
%3 = addf %2#0, %2#1 : f32
```

as

```
auto results =
    LoopNestBuilder(&i, start, end, step, {&arg0, &arg1},  {init0, init1})([&] {
      auto sum = arg0 + arg1;
      loop_yield(ArrayRef<ValueHandle>{arg1, sum});
    });

// Add the two values accumulated by the yield-for-loop:
ValueHandle(results[0]) + ValueHandle(results[1]);
```

Differential Revision: https://reviews.llvm.org/D78093