Matthias Springer 76435f2dca [mlir][SCF] ValueBoundsConstraintSet: Support scf.if (branches) (#87860) ...

This commit adds support for `scf.if` to `ValueBoundsConstraintSet`.

Example:
```
%0 = scf.if ... -> index {
  scf.yield %a : index
} else {
  scf.yield %b : index
}
```

The following constraints hold for %0:
* %0 >= min(%a, %b)
* %0 <= max(%a, %b)

Such constraints cannot be added to the constraint set; min/max is not
supported by `IntegerRelation`. However, if we know which one of %a and
%b is larger, we can add constraints for %0. E.g., if %a <= %b:
* %0 >= %a
* %0 <= %b

This commit required a few minor changes to the
`ValueBoundsConstraintSet` infrastructure, so that values can be
compared while we are still in the process of traversing the IR/adding
constraints.

Note: This is a re-upload of #85895, which was reverted. The bug that
caused the failure was fixed in #87859.

2024-04-06 13:04:49 +09:00

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