Implement a simple register allocator that tries to keep instruction results in registers.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
diff --git a/runtime/vm/assembler_x64.cc b/runtime/vm/assembler_x64.cc
index 1d0884975aa04a7a819e6ba99de324ada5dd8ac6..729a3b18a2c0372ed52989702d44fba3f7df8372 100644
--- a/runtime/vm/assembler_x64.cc
+++ b/runtime/vm/assembler_x64.cc
@@ -1327,6 +1327,16 @@ void Assembler::MoveRegister(Register to, Register from) {
 }
 
 
+void Assembler::PushRegister(Register r) {
+  pushq(r);
+}
+
+
+void Assembler::PopRegister(Register r) {
+  popq(r);
+}
+
+
 void Assembler::AddImmediate(Register reg, const Immediate& imm) {
   int64_t value = imm.value();
   if (value > 0) {