Refactor bigint shifting code in preparation of shifting intrinsics.

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
===================================================================
--- runtime/vm/intrinsifier_x64.cc	(revision 44750)
+++ runtime/vm/intrinsifier_x64.cc	(working copy)
@@ -725,6 +725,16 @@
 }
 
 
+void Intrinsifier::Bigint_lsh(Assembler* assembler) {
+  // TODO(regis): Implement.
+}
+
+
+void Intrinsifier::Bigint_rsh(Assembler* assembler) {
+  // TODO(regis): Implement.
+}
+
+
 void Intrinsifier::Bigint_absAdd(Assembler* assembler) {
   // static void _absAdd(Uint32List digits, int used,
   //                     Uint32List a_digits, int a_used,