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

[Florian Hahn via llvm-commits]

================
@@ -1696,3 +1696,94 @@ bool InductionDescriptor::isInductionPHI(
                           /*InductionBinOp=*/nullptr, /*Casts=*/nullptr, Preds);
   return true;
 }
+
+// Recognize a monotonic PHI variable by matching the following pattern:
+// loop_header:
+//   %monotonic_phi = phi [ %start, %preheader ], [ %latch_phi, %latch ]
+//   br i1 %do_step, label %step_bb, label %latch
+//
+// step_bb:
+//   %step = add/gep %monotonic_phi, %step_val
+//   br label %latch
+//
+// latch:
+//   %latch_phi = phi [ %monotonic_phi, %loop_header ], [ %step, %step_bb ]
+//   br label %loop_header
+bool MonotonicDescriptor::isMonotonicPHI(PHINode *HeaderPHI, const Loop *L,
----------------
fhahn wrote:

```suggestion
bool MonotonicDescriptor::isMonotonicPHI(PHINode *PN, const Loop *L,
```

for consistency with declaration

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