[llvm] [InstCombine] Implement folds of icmp of UCMP/SCMP call and a constant (PR #96118)

[Yingwei Zheng via llvm-commits]

================
@@ -3926,6 +3926,52 @@ foldICmpUSubSatOrUAddSatWithConstant(ICmpInst::Predicate Pred,
       ConstantInt::get(Op1->getType(), EquivInt));
 }
 
+static Instruction *
+foldICmpOfCmpIntrinsicWithConstant(ICmpInst::Predicate Pred, IntrinsicInst *I,
+                                   const APInt &C,
+                                   InstCombiner::BuilderTy &Builder) {
+  std::optional<ICmpInst::Predicate> NewPredicate = std::nullopt;
+  switch (Pred) {
+  case ICmpInst::ICMP_EQ:
+  case ICmpInst::ICMP_NE:
+    if (C.isZero())
+      NewPredicate = Pred;
+    if (C.isOne())
+      NewPredicate =
+          Pred == ICmpInst::ICMP_EQ ? ICmpInst::ICMP_UGT : ICmpInst::ICMP_ULE;
+    if (C.isAllOnes())
+      NewPredicate =
+          Pred == ICmpInst::ICMP_EQ ? ICmpInst::ICMP_ULT : ICmpInst::ICMP_UGE;
----------------
dtcxzyw wrote:

```suggestion
    else if (C.isOne())
      NewPredicate =
          Pred == ICmpInst::ICMP_EQ ? ICmpInst::ICMP_UGT : ICmpInst::ICMP_ULE;
    else if (C.isAllOnes())
      NewPredicate =
          Pred == ICmpInst::ICMP_EQ ? ICmpInst::ICMP_ULT : ICmpInst::ICMP_UGE;
```

https://github.com/llvm/llvm-project/pull/96118