[llvm] [ValueTracking] Add `matchSimpleBinaryIntrinsicRecurrence` helper (PR #145964)

[Nikita Popov via llvm-commits]

================
@@ -965,6 +966,21 @@ LLVM_ABI bool matchSimpleRecurrence(const PHINode *P, BinaryOperator *&BO,
 LLVM_ABI bool matchSimpleRecurrence(const BinaryOperator *I, PHINode *&P,
                                     Value *&Start, Value *&Step);
 
+/// Attempt to match a simple value-accumulating recurrence of the form:
+///   %llvm.intrinsic.acc = phi Ty [%Init, %Entry], [%llvm.intrinsic, %backedge]
+///   %llvm.intrinsic = call Ty @llvm.intrinsic(%OtherOp, %llvm.intrinsic.acc)
+/// OR
+///   %llvm.intrinsic.acc = phi Ty [%Init, %Entry], [%llvm.intrinsic, %backedge]
+///   %llvm.intrinsic = call Ty @llvm.intrinsic(%llvm.intrinsic.acc, %OtherOp)
+///
+/// The recurrence relation is of kind:
+///   X_0 = %a (initial value),
+///   X_i = call @llvm.binary.intrinsic(X_i-1, %b)
+/// Where %b may be a loop-invariant value.
----------------
nikic wrote:

I still find the phrasing here confusing...
```suggestion
/// Where %b is not required to be loop-invariant.
```

https://github.com/llvm/llvm-project/pull/145964