[llvm] [CVP]: Fold `icmp eq X, C` to `trunc X to i1` if C=2k+1 and X in [2k, 2k+1] (PR #83829)

[Yingwei Zheng via llvm-commits]

================
@@ -332,6 +332,39 @@ static bool constantFoldCmp(CmpInst *Cmp, LazyValueInfo *LVI) {
   return true;
 }
 
+/// Given an icmp `icmp eq X, C`,
+/// if we already know that C is 2k+1 and X is in [2k, 2k+1],
+/// then we can fold it to `trunc X to i1`.
+static bool processEqualityICmp(CmpInst *Cmp, LazyValueInfo *LVI) {
+  if (Cmp->getType()->isVectorTy() ||
+      !Cmp->getOperand(0)->getType()->isIntegerTy() || !Cmp->isEquality())
+    return false;
+
+  Value *Op0 = Cmp->getOperand(0);
+  Value *Op1 = Cmp->getOperand(1);
+  ConstantInt *CI = dyn_cast<ConstantInt>(Op1);
+  if (!CI)
+    return false;
----------------
dtcxzyw wrote:

```suggestion
  const APInt *RHSC;
  if (!match(Cmp->getOperand(1), m_APInt(RHSC)))
    return false;
```

https://github.com/llvm/llvm-project/pull/83829