On x64 use 32bit idiv when possible instead of 64bit one.

runtime/vm/assembler_x64_test.cc

Index: runtime/vm/assembler_x64_test.cc
diff --git a/runtime/vm/assembler_x64_test.cc b/runtime/vm/assembler_x64_test.cc
index 1287571390502b17f76b85497edbc4bc013172fe..2d29d9debb23edbe3be363c49f8d0bec87f913a0 100644
--- a/runtime/vm/assembler_x64_test.cc
+++ b/runtime/vm/assembler_x64_test.cc
@@ -515,6 +515,22 @@ ASSEMBLER_TEST_RUN(MoveExtend, test) {
 }
 
 
+ASSEMBLER_TEST_GENERATE(MoveExtend32, assembler) {
+  __ movq(RDX, Immediate(0xffffffff));
+  __ movsxd(RDX, RDX);
+  __ movq(RAX, Immediate(0x7fffffff));
+  __ movsxd(RAX, RAX);
+  __ addq(RAX, RDX);
+  __ ret();
+}
+
+
+ASSEMBLER_TEST_RUN(MoveExtend32, test) {
+  typedef intptr_t (*MoveExtend)();
+  EXPECT_EQ(0x7ffffffe, reinterpret_cast<MoveExtend>(test->entry())());
+}
+
+
 ASSEMBLER_TEST_GENERATE(MoveExtendMemory, assembler) {
   __ movq(RDX, Immediate(0x123456781234ffff));
 
@@ -537,6 +553,24 @@ ASSEMBLER_TEST_RUN(MoveExtendMemory, test) {
 }
 
 
+ASSEMBLER_TEST_GENERATE(MoveExtend32Memory, assembler) {
+  __ pushq(Immediate(0xffffffff));
+  __ pushq(Immediate(0x7fffffff));
+  __ movsxd(RDX, Address(RSP, kWordSize));
+  __ movsxd(RAX, Address(RSP, 0));
+  __ addq(RSP, Immediate(kWordSize * 2));
+
+  __ addq(RAX, RDX);
+  __ ret();
+}
+
+
+ASSEMBLER_TEST_RUN(MoveExtend32Memory, test) {
+  typedef intptr_t (*MoveExtend)();
+  EXPECT_EQ(0x7ffffffe, reinterpret_cast<MoveExtend>(test->entry())());
+}
+
+
 ASSEMBLER_TEST_GENERATE(MoveWord, assembler) {
   __ xorq(RAX, RAX);
   __ pushq(Immediate(0));