[llvm] [InstCombine] Add missing patterns for scmp and ucmp (PR #149225)

[Yingwei Zheng via llvm-commits]

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@@ -3849,6 +3853,45 @@ Instruction *InstCombinerImpl::foldSelectToCmp(SelectInst &SI) {
     }
   }
 
+  // Special cases with constants: x == C ? 0 : (x > C-1 ? 1 : -1)
+  if (Pred == ICmpInst::ICMP_EQ && match(TV, m_Zero())) {
+    Value *X;
+    const APInt *C;
+    if (match(LHS, m_Value(X)) && match(RHS, m_APInt(C))) {
+      CmpPredicate InnerPred;
+      Value *InnerLHS, *InnerRHS;
+      const APInt *InnerTV, *InnerFV;
+      if (match(FV, m_Select(
+                        m_ICmp(InnerPred, m_Value(InnerLHS), m_Value(InnerRHS)),
+                        m_APInt(InnerTV), m_APInt(InnerFV)))) {
+
+        // x == C ? 0 : (x > C-1 ? 1 : -1)
+        if (ICmpInst::isGT(InnerPred) && InnerTV->isOne() &&
+            InnerFV->isAllOnes()) {
+          IsSigned = ICmpInst::isSigned(InnerPred);
+          bool CanSubOne = IsSigned ? !C->isMinSignedValue() : !C->isMinValue();
+          if (CanSubOne) {
+            APInt Cminus1 = *C - 1;
+            if (InnerLHS == X && match(InnerRHS, m_SpecificInt(Cminus1)))
----------------
dtcxzyw wrote:

```suggestion
            if (match(InnerRHS, m_SpecificInt(Cminus1)))
```

https://github.com/llvm/llvm-project/pull/149225