[llvm] [IVDescriptors] Implement MonotonicDescriptor (PR #214490)

[Florian Hahn via llvm-commits]

================
@@ -482,6 +482,68 @@ class InductionDescriptor {
   SmallVector<const SCEVPredicate *, 2> NoWrapPredicates;
 };
 
+/// A struct for saving information about monotonic variables.
+/// Monotonic variable can be considered as a "conditional" induction variable:
+/// its update happens only on loop iterations for which a certain predicate is
+/// satisfied. The step of the monotonic variable must be loop-invariant.
+class MonotonicDescriptor {
+public:
+  MonotonicDescriptor() = default;
+
+  /// Returns true if \p PN is a monotonic variable in the loop \p L. If \p PN
+  /// is monotonic, the monotonic descriptor \p D will contain the data
----------------
fhahn wrote:

```suggestion
  /// is monotonic, the monotonic descriptor \p Desc will contain the data
```

https://github.com/llvm/llvm-project/pull/214490