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

[Benjamin Maxwell via llvm-commits]

================
@@ -1696,3 +1696,125 @@ bool InductionDescriptor::isInductionPHI(
                           /*InductionBinOp=*/nullptr, /*Casts=*/nullptr, Preds);
   return true;
 }
+
+bool MonotonicDescriptor::setSCEV(const SCEV *NewExpr) {
+  auto *AddRec = dyn_cast<SCEVAddRecExpr>(NewExpr);
+  if (!AddRec || !AddRec->isAffine())
+    return false;
+  Expr = AddRec;
+  return true;
+}
+
+// Recognize monotonic phi variable by matching the following pattern:
+// loop_header:
+//   %monotonic_phi = [%start, %preheader], [%chain_phi0, %latch]
+//
+// step_bb:
+//   %step = add/gep %monotonic_phi, %step_val
+//
+// bbN:
+//   %chain_phiN = [%monotonic_phi, ], [%step, ]
+//
+// ...
+//
+// bb1:
+//   %chain_phi1 = [%monotonic_phi, ], [%chain_phi2, ]
+//
+// latch:
+//   %chain_phi0 = [%monotonic_phi, %pred], [%chain_phi1, %pred]
+//
+// For this pattern, monotonic phi is described by {%start, +, %step} recurrence
+// and predicate is CFG edge %step_bb -> %bbN.
+bool MonotonicDescriptor::isMonotonicPHI(PHINode *PN, const Loop *L,
+                                         MonotonicDescriptor &Desc,
+                                         ScalarEvolution &SE) {
----------------
MacDue wrote:

> AFAIK this is similar to what is done for RecurrenceDescriptor.

The standard entry point for `RecurrenceDescriptor` is still / `AddReductionVar`, which takes the descriptor by reference, e.g.: 

```c++
LLVM_ABI static bool
  isReductionPHI(PHINode *Phi, Loop *TheLoop, RecurrenceDescriptor &RedDes,
                 DemandedBits *DB = nullptr, AssumptionCache *AC = nullptr,
                 DominatorTree *DT = nullptr, ScalarEvolution *SE = nullptr);
```

The main difference is it assigns via a constructor:

```
  RedDes =
      RecurrenceDescriptor(RdxStart, ExitInstruction, IntermediateStore, Kind,
                           FMF, ExactFPMathInst, RecurrenceType, IsSigned,
                           IsOrdered, CastInsts, MinWidthCastToRecurrenceType);
```

Which I could refactor this to do.

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