[llvm] [InstCombine] Simplify the pattern `a ne/eq (zext (a ne/eq c))` (PR #65852)

[Nikita Popov via llvm-commits]

================
@@ -6309,7 +6309,69 @@ Instruction *InstCombinerImpl::foldICmpUsingBoolRange(ICmpInst &I) {
       Y->getType()->isIntOrIntVectorTy(1) && Pred == ICmpInst::ICMP_ULE)
     return BinaryOperator::CreateOr(Builder.CreateIsNull(X), Y);
 
+  ICmpInst::Predicate Pred1, Pred2;
   const APInt *C;
+  // icmp eq/ne X, (zext (icmp eq/ne X, C))
+  if (match(&I, m_c_ICmp(Pred1, m_Value(X),
+                         m_ZExt(m_ICmp(Pred2, m_Deferred(X), m_APInt(C))))) &&
+      ICmpInst::isEquality(Pred1) && ICmpInst::isEquality(Pred2)) {
+    if (C->isZero()) {
+      if (Pred2 == ICmpInst::ICMP_EQ) {
+        // icmp eq X, (zext (icmp eq X, 0)) --> false
+        // icmp ne X, (zext (icmp eq X, 0)) --> true
+        return replaceInstUsesWith(
+            I,
+            Constant::getIntegerValue(
+                I.getType(),
+                APInt(1U, static_cast<uint64_t>(Pred1 == ICmpInst::ICMP_NE))));
----------------
nikic wrote:

```suggestion
            ConstantInt::getBool(
                I.getType(), Pred1 == ICmpInst::ICMP_NE)));
```
etc

https://github.com/llvm/llvm-project/pull/65852