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

[Luke Lau via llvm-commits]

================
@@ -482,6 +483,67 @@ 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;
+
+  MonotonicDescriptor(PHINode *HeaderPHI, PHINode *BackedgePHI,
+                      Instruction *StepInst, const SCEV *StartSCEV,
+                      const SCEV *StepSCEV, unsigned SCEVNoWrapFlags)
+      : HeaderPHI(HeaderPHI), BackedgePHI(BackedgePHI), StepInst(StepInst),
+        StartSCEV(StartSCEV), StepSCEV(StepSCEV),
+        SCEVNoWrapFlags(SCEVNoWrapFlags) {}
+
+  /// 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
+  /// describing the PHI.
+  static bool isMonotonicPHI(PHINode *PN, const Loop *L,
----------------
lukel97 wrote:

I think this needs 
```suggestion
  LLVM_ABI static bool isMonotonicPHI(PHINode *PN, const Loop *L,
```

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