[llvm] [InstCombine] Fold select chain to frozen bool reduction (PR #217482)

[Simon Pilgrim via llvm-commits]

================
@@ -4582,6 +4582,65 @@ static bool isSelectZeroSignInsignificant(SelectInst &SI) {
   return true;
 }
 
+static Instruction *foldSelectExtractEl(SelectInst &SI,
+                                        InstCombiner::BuilderTy &Builder) {
+  //       select (extractelement (icmp Pred X, Y), C1)
+  //              (extractelement (icmp Pred X, Y) C2), 0
+  //              ->
+  //              icmp eq (bitcast (freeze (icmp Pred X, Y)) to iN), C3
+  Value *X;
+  Value *Y;
+  CmpPredicate Pred1, Pred2;
+  const APInt *C1, *C2;
+  if (match(&SI,
+            m_Select(m_ExtractElt(m_ICmp(Pred1, m_Value(X), m_Value(Y)),
+                                  m_APInt(C1)),
+                     m_ExtractElt(m_ICmp(Pred2, m_Deferred(X), m_Deferred(Y)),
+                                  m_APInt(C2)),
+                     m_Zero()))
----------------
RKSimon wrote:

(style) early out:
```
  if (!match(&SI,
            m_Select(m_ExtractElt(m_ICmp(Pred1, m_Value(X), m_Value(Y)), m_APInt(C1)),
                     m_ExtractElt(m_ICmp(Pred2, m_Deferred(X), m_Deferred(Y)), m_APInt(C2)),
                     m_Zero()) ||
      !CmpPredicate::getMatching(Pred1, Pred2);)
    return nullptr;
```

https://github.com/llvm/llvm-project/pull/217482