[SIL] Marked visitTransitiveIncomingPhis const.

Author: nate-chandler

include/swift/SIL/SILArgument.h

@@ -291,7 +291,7 @@ class SILPhiArgument : public SILArgument {
   ///
   /// Returns false when called on a non-phi and when the visitor returns false.
   bool visitTransitiveIncomingPhiOperands(
-      function_ref<bool(SILPhiArgument *, Operand *)> visitor);
+      function_ref<bool(SILPhiArgument *, Operand *)> visitor) const;
 
   /// 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

lib/SIL/IR/SILArgument.cpp

@@ -227,12 +227,12 @@ bool SILPhiArgument::getIncomingPhiValues(
 }
 
 bool SILPhiArgument::visitTransitiveIncomingPhiOperands(
-    function_ref<bool(SILPhiArgument *, Operand *)> visitor) {
+    function_ref<bool(SILPhiArgument *, Operand *)> visitor) const {
   if (!isPhi())
     return false;
 
   GraphNodeWorklist<SILPhiArgument *, 4> worklist;
-  worklist.initialize(this);
+  worklist.insert(const_cast<SILPhiArgument *>(this));
 
   while (auto *argument = worklist.pop()) {
     SmallVector<Operand *> operands;