Add a const method to iterator over PHI incoming values.

This will be used in the next commit which converts code to use incoming_values().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237168 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Instructions.h

@@ -2278,6 +2278,8 @@ public:
 
   op_range incoming_values() { return operands(); }
 
+  const_op_range incoming_values() const { return operands(); }
+
   /// getNumIncomingValues - Return the number of incoming edges
   ///
   unsigned getNumIncomingValues() const { return getNumOperands(); }