[mlir:SymbolTable] Add "remove" method that drops a symbol without erasing it

[rriddle]

Summary

There are various use cases where we don't want to immediately erase
an operation when removing it from the symbol table. This commit adds
a "remove" method to support that.

[lattner]

Makes sense!

mlir/include/mlir/IR/SymbolTable.h

public:
   /// Look up a symbol with the specified name, returning null if no such
   /// name exists. Names never include the @ on them.
   Operation *lookup(StringAttr name) const;
   template <typename T>
   T lookup(StringAttr name) const {
     return dyn_cast_or_null<T>(lookup(name));
   }
 
-  /// Erase the given symbol from the table.
+  /// Remove the given symbol from the table, without deleting it.
+  void remove(Operation *op);
+
+  /// Erase the given symbol from the table and delete the operation.
   void erase(Operation *symbol);
 
   /// Insert a new symbol into the table, and rename it as necessary to avoid
   /// collisions. Also insert at the specified location in the body of the
   /// associated operation if it is not already there. It is asserted that the
   /// symbol is not inside another operation. Return the name of the symbol
   /// after insertion as attribute.
   StringAttr insert(Operation *symbol, Block::iterator insertPt = {});

mlir/lib/IR/SymbolTable.cpp

 /// exists. Names never include the @ on them.
 Operation *SymbolTable::lookup(StringRef name) const {
   return lookup(StringAttr::get(symbolTableOp->getContext(), name));
 }
 Operation *SymbolTable::lookup(StringAttr name) const {
   return symbolTable.lookup(name);
 }
 
-/// Erase the given symbol from the table.
-void SymbolTable::erase(Operation *symbol) {
-  StringAttr name = getNameIfSymbol(symbol);
+void SymbolTable::remove(Operation *op) {
+  StringAttr name = getNameIfSymbol(op);
   assert(name && "expected valid 'name' attribute");
-  assert(symbol->getParentOp() == symbolTableOp &&
+  assert(op->getParentOp() == symbolTableOp &&
          "expected this operation to be inside of the operation with this "
          "SymbolTable");
 
   auto it = symbolTable.find(name);
-  if (it != symbolTable.end() && it->second == symbol) {
+  if (it != symbolTable.end() && it->second == op)
     symbolTable.erase(it);
-    symbol->erase();
-  }
+
+void SymbolTable::erase(Operation *symbol) {
+  remove(symbol);
+  symbol->erase();
 }
 
 // TODO: Consider if this should be renamed to something like insertOrUpdate
 /// Insert a new symbol into the table and associated operation if not already
 /// there and rename it as necessary to avoid collisions. Return the name of
 /// the symbol after insertion as attribute.
 StringAttr SymbolTable::insert(Operation *symbol, Block::iterator insertPt) {
   // The symbol cannot be the child of another op and must be the child of the