[llvm] [InstCombine] Fold `(x < y) ? -1 : zext(x != y)` into `u/scmp(x,y)` (PR #101049)

[Yingwei Zheng via llvm-commits]

================
@@ -3529,6 +3529,38 @@ static Instruction *foldBitCeil(SelectInst &SI, IRBuilderBase &Builder) {
                                 Masked);
 }
 
+// This function tries to fold the following sequence
+//   %lt = icmp ult/slt i32 %x, %y
+//   %ne0 = icmp ne i32 %x, %y
+//   %ne = zext i1 %ne0 to iN
+//   %r = select i1 %lt, iN -1, iN %ne
+// into
+//   %r = call iN @llvm.ucmp/scmp(%x, %y)
+Instruction *InstCombinerImpl::foldSelectToCmp(SelectInst &SI) {
+  if (!isa<Constant>(SI.getTrueValue()) ||
+      !dyn_cast<Constant>(SI.getTrueValue())->isAllOnesValue())
+    return nullptr;
+
+  Value *LHS, *RHS;
+  ICmpInst::Predicate NEPred;
+  if (!match(SI.getFalseValue(),
+             m_ZExt(m_ICmp(NEPred, m_Value(LHS), m_Value(RHS)))) ||
+      NEPred != ICmpInst::ICMP_NE)
----------------
dtcxzyw wrote:

```suggestion
             m_ZExt(m_SpecificICmp(ICmpInst::ICMP_NE, m_Value(LHS), m_Value(RHS)))))
```

https://github.com/llvm/llvm-project/pull/101049