[SCEV] Move SCEVPoisonCollector outside function (NFC)

To allow reusing it. Also switch it to store SCEVUnknown rather
than SCEV, as only these can produce poison.

Author: nikic

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -4108,36 +4108,38 @@ static bool scevUnconditionallyPropagatesPoisonFromOperands(SCEVTypes Kind) {
   llvm_unreachable("Unknown SCEV kind!");
 }
 
-/// Return true if V is poison given that AssumedPoison is already poison.
-static bool impliesPoison(const SCEV *AssumedPoison, const SCEV *S) {
-  // The only way poison may be introduced in a SCEV expression is from a
-  // poison SCEVUnknown (ConstantExprs are also represented as SCEVUnknown,
-  // not SCEVConstant). Notably, nowrap flags in SCEV nodes can *not*
-  // introduce poison -- they encode guaranteed, non-speculated knowledge.
-  //
-  // Additionally, all SCEV nodes propagate poison from inputs to outputs,
-  // with the notable exception of umin_seq, where only poison from the first
-  // operand is (unconditionally) propagated.
-  struct SCEVPoisonCollector {
-    bool LookThroughMaybePoisonBlocking;
-    SmallPtrSet<const SCEV *, 4> MaybePoison;
-    SCEVPoisonCollector(bool LookThroughMaybePoisonBlocking)
-        : LookThroughMaybePoisonBlocking(LookThroughMaybePoisonBlocking) {}
-
-    bool follow(const SCEV *S) {
-      if (!LookThroughMaybePoisonBlocking &&
-          !scevUnconditionallyPropagatesPoisonFromOperands(S->getSCEVType()))
-        return false;
+namespace {
+// The only way poison may be introduced in a SCEV expression is from a
+// poison SCEVUnknown (ConstantExprs are also represented as SCEVUnknown,
+// not SCEVConstant). Notably, nowrap flags in SCEV nodes can *not*
+// introduce poison -- they encode guaranteed, non-speculated knowledge.
+//
+// Additionally, all SCEV nodes propagate poison from inputs to outputs,
+// with the notable exception of umin_seq, where only poison from the first
+// operand is (unconditionally) propagated.
+struct SCEVPoisonCollector {
+  bool LookThroughMaybePoisonBlocking;
+  SmallPtrSet<const SCEVUnknown *, 4> MaybePoison;
+  SCEVPoisonCollector(bool LookThroughMaybePoisonBlocking)
+      : LookThroughMaybePoisonBlocking(LookThroughMaybePoisonBlocking) {}
+
+  bool follow(const SCEV *S) {
+    if (!LookThroughMaybePoisonBlocking &&
+        !scevUnconditionallyPropagatesPoisonFromOperands(S->getSCEVType()))
+      return false;
 
-      if (auto *SU = dyn_cast<SCEVUnknown>(S)) {
-        if (!isGuaranteedNotToBePoison(SU->getValue()))
-          MaybePoison.insert(S);
-      }
-      return true;
+    if (auto *SU = dyn_cast<SCEVUnknown>(S)) {
+      if (!isGuaranteedNotToBePoison(SU->getValue()))
+        MaybePoison.insert(SU);
     }
-    bool isDone() const { return false; }
-  };
+    return true;
+  }
+  bool isDone() const { return false; }
+};
+} // namespace
 
+/// Return true if V is poison given that AssumedPoison is already poison.
+static bool impliesPoison(const SCEV *AssumedPoison, const SCEV *S) {
   // First collect all SCEVs that might result in AssumedPoison to be poison.
   // We need to look through potentially poison-blocking operations here,
   // because we want to find all SCEVs that *might* result in poison, not only
@@ -4158,8 +4160,9 @@ static bool impliesPoison(const SCEV *AssumedPoison, const SCEV *S) {
 
   // Make sure that no matter which SCEV in PC1.MaybePoison is actually poison,
   // it will also make S poison by being part of PC2.MaybePoison.
-  return all_of(PC1.MaybePoison,
-                [&](const SCEV *S) { return PC2.MaybePoison.contains(S); });
+  return all_of(PC1.MaybePoison, [&](const SCEVUnknown *S) {
+    return PC2.MaybePoison.contains(S);
+  });
 }
 
 const SCEV *