[MemAccessUtils] Added PathNode::isPrefixOf.

It's just a passthrough to the same method on IndexTrieNode.

Author: nate-chandler

include/swift/SIL/MemAccessUtils.h

@@ -1132,6 +1132,8 @@ class AccessPath {
     // Precondition: this != subNode
     PathNode findPrefix(PathNode subNode) const;
 
+    bool isPrefixOf(PathNode other) { return node->isPrefixOf(other.node); }
+
     bool operator==(PathNode other) const { return node == other.node; }
     bool operator!=(PathNode other) const { return node != other.node; }
   };