[MLIR] Apply clang-tidy fixes for misc-use-internal-linkage in PredicateTree.cpp (NFC)

Author: joker-eph

mlir/lib/Conversion/PDLToPDLInterp/PredicateTree.cpp

@@ -800,9 +800,9 @@ static bool isSamePredicate(MatcherNode *node, OrderedPredicate *predicate) {
 
 /// Get or insert a child matcher for the given parent switch node, given a
 /// predicate and parent pattern.
-std::unique_ptr<MatcherNode> &getOrCreateChild(SwitchNode *node,
-                                               OrderedPredicate *predicate,
-                                               pdl::PatternOp pattern) {
+static std::unique_ptr<MatcherNode> &
+getOrCreateChild(SwitchNode *node, OrderedPredicate *predicate,
+                 pdl::PatternOp pattern) {
   assert(isSamePredicate(node, predicate) &&
          "expected matcher to equal the given predicate");