Introduce a VM type cast to avoid repeating a type test and spare a handle.

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
===================================================================
--- runtime/vm/flow_graph_compiler_x64.cc	(revision 9203)
+++ runtime/vm/flow_graph_compiler_x64.cc	(working copy)
@@ -312,9 +312,7 @@
   const Immediate raw_null =
       Immediate(reinterpret_cast<intptr_t>(Object::null()));
   if (type.IsTypeParameter()) {
-    // TODO(regis): Introduce and use TypeParameter::Cast().
-    const TypeParameter* type_param =
-        reinterpret_cast<const TypeParameter*>(&type);
+    const TypeParameter* type_param = TypeParameter::Cast(type);
     // Load instantiator (or null) and instantiator type arguments on stack.
     __ movq(RDX, Address(RSP, 0));  // Get instantiator type arguments.
     // RDX: instantiator type arguments.