[SIL] Add utility to transitively visit operands.

The new utility looks through operands whose values are themselves
SILPhiArguments and visits those arguments' operands.

Author: nate-chandler

include/swift/SIL/SILArgument.h

@@ -273,6 +273,13 @@ class SILPhiArgument : public SILArgument {
   /// If visitor returns false, iteration is stopped and we return false.
   bool visitIncomingPhiOperands(function_ref<bool(Operand *)> visitor) const;
 
+  /// Visit incoming phi operands and the argument into which they are incoming;
+  /// if an operand's value is itself a phi, visit that phi's operands.
+  ///
+  /// Returns false when called on a non-phi and when the visitor returns false.
+  bool visitTransitiveIncomingPhiOperands(
+      function_ref<bool(SILPhiArgument *, Operand *)> visitor);
+
   /// Returns true if we were able to find a single terminator operand value for
   /// each predecessor of this arguments basic block. The found values are
   /// stored in OutArray.

lib/SIL/IR/SILArgument.cpp

@@ -10,12 +10,13 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/ADT/STLExtras.h"
-#include "swift/SIL/SILBasicBlock.h"
 #include "swift/SIL/SILArgument.h"
+#include "swift/Basic/GraphNodeWorklist.h"
+#include "swift/SIL/SILBasicBlock.h"
 #include "swift/SIL/SILFunction.h"
 #include "swift/SIL/SILInstruction.h"
 #include "swift/SIL/SILModule.h"
+#include "llvm/ADT/STLExtras.h"
 
 using namespace swift;
 
@@ -226,6 +227,31 @@ bool SILPhiArgument::getIncomingPhiValues(
   return true;
 }
 
+bool SILPhiArgument::visitTransitiveIncomingPhiOperands(
+    function_ref<bool(SILPhiArgument *, Operand *)> visitor) {
+  if (!isPhi())
+    return false;
+
+  GraphNodeWorklist<SILPhiArgument *, 4> worklist;
+  worklist.initialize(this);
+
+  while (auto *argument = worklist.pop()) {
+    llvm::SmallVector<Operand *> operands;
+    argument->getIncomingPhiOperands(operands);
+
+    for (auto *operand : operands) {
+      SILPhiArgument *forwarded;
+      if ((forwarded = dyn_cast<SILPhiArgument>(operand->get())) &&
+          forwarded->isPhi()) {
+        worklist.insert(forwarded);
+      }
+      if (!visitor(argument, operand))
+        return false;
+    }
+  }
+  return true;
+}
+
 static SILValue
 getSingleTerminatorOperandForPred(const SILBasicBlock *parentBlock,
                                   const SILBasicBlock *predBlock,