[NFC][MLIR] Simplify lookup of nested symbols (#179362)

Consolidate the handling of nested symbols in the loop over nested
references. This is an NFC refactor to make it simpler to enforce symbol
visibility here.

mlir/lib/IR/SymbolTable.cpp

diff --git a/mlir/lib/IR/SymbolTable.cpp b/mlir/lib/IR/SymbolTable.cpp
index 9f5dd2c..4f8418f 100644
--- a/mlir/lib/IR/SymbolTable.cpp
+++ b/mlir/lib/IR/SymbolTable.cpp
@@ -413,30 +413,22 @@
   assert(symbolTableOp->hasTrait<OpTrait::SymbolTable>());
 
   // Lookup the root reference for this symbol.
-  symbolTableOp = lookupSymbolFn(symbolTableOp, symbol.getRootReference());
-  if (!symbolTableOp)
+  auto *symbolOp = lookupSymbolFn(symbolTableOp, symbol.getRootReference());
+  if (!symbolOp)
     return failure();
-  symbols.push_back(symbolTableOp);
+  symbols.push_back(symbolOp);
 
-  // If there are no nested references, just return the root symbol directly.
-  ArrayRef<FlatSymbolRefAttr> nestedRefs = symbol.getNestedReferences();
-  if (nestedRefs.empty())
-    return success();
-
-  // Verify that the root is also a symbol table.
-  if (!symbolTableOp->hasTrait<OpTrait::SymbolTable>())
-    return failure();
-
-  // Otherwise, lookup each of the nested non-leaf references and ensure that
-  // each corresponds to a valid symbol table.
-  for (FlatSymbolRefAttr ref : nestedRefs.drop_back()) {
-    symbolTableOp = lookupSymbolFn(symbolTableOp, ref.getAttr());
-    if (!symbolTableOp || !symbolTableOp->hasTrait<OpTrait::SymbolTable>())
+  // Lookup each of the nested references.
+  for (FlatSymbolRefAttr ref : symbol.getNestedReferences()) {
+    // Check that we have a valid symbol table to lookup ref.
+    if (!symbolOp->hasTrait<OpTrait::SymbolTable>())
       return failure();
-    symbols.push_back(symbolTableOp);
+    symbolOp = lookupSymbolFn(symbolOp, ref.getAttr());
+    if (!symbolOp)
+      return failure();
+    symbols.push_back(symbolOp);
   }
-  symbols.push_back(lookupSymbolFn(symbolTableOp, symbol.getLeafReference()));
-  return success(symbols.back());
+  return success();
 }
 
 LogicalResult