[MLIR][SCF] Fix nested if merging bug

The current nested if merging has a bug. Specifically, consider the following code:

```
    %r = scf.if %arg3 -> (i32) {
      scf.if %arg1 {
        "test.op"() : () -> ()
      }
      scf.yield %arg0 : i32
    } else {
      scf.yield %arg2 : i32
    }
```

When the above gets merged, it will become:
```
    %r = scf.if %arg3 && %arg1-> (i32) {
      "test.op"() : () -> ()
      scf.yield %arg0 : i32
    } else {
      scf.yield %arg2 : i32
    }
```

However, this means that when only %arg3 is true, we will incorrectly return %arg2 instead
of %arg0. This change updates the behavior of the pass to only enable nested if merging where
the outer yield contains only values from the inner if, or values defined outside of the if.

In the case of the latter, they can turned into a select of only the outer if condition, thus
maintaining correctness.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D122108