[vm/assembler] Add IMUL with implicit operands

Rationale:
Needed for an upcoming improvement of DIV/MOD by constant.

#37789

Change-Id: I3966049a3f14fd3becec797c4bc0038f2852bc1b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/114680
Commit-Queue: Aart Bik <ajcbik@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>

runtime/vm/compiler/assembler/assembler_x64.h

@@ -590,6 +590,7 @@ class Assembler : public AssemblerBase {
   REGULAR_UNARY(not, 0xF7, 2)
   REGULAR_UNARY(neg, 0xF7, 3)
   REGULAR_UNARY(mul, 0xF7, 4)
+  REGULAR_UNARY(imul, 0xF7, 5)
   REGULAR_UNARY(div, 0xF7, 6)
   REGULAR_UNARY(idiv, 0xF7, 7)
   REGULAR_UNARY(inc, 0xFF, 0)

runtime/vm/compiler/assembler/assembler_x64_test.cc

@@ -855,6 +855,25 @@ ASSEMBLER_TEST_RUN(UnsignedMultiply, test) {
       "ret\n");
 }
 
+ASSEMBLER_TEST_GENERATE(SignedMultiply64Implicit, assembler) {
+  __ movq(RAX, Immediate(7));
+  __ movq(RDX, Immediate(-3));
+  __ imulq(RDX);  // // RDX:RAX = -21
+  __ addq(RAX, RDX);
+  __ ret();
+}
+
+ASSEMBLER_TEST_RUN(SignedMultiply64Implicit, test) {
+  typedef int (*SignedMultiply64Implicit)();
+  EXPECT_EQ(-22, reinterpret_cast<SignedMultiply64Implicit>(test->entry())());
+  EXPECT_DISASSEMBLY(
+      "movl rax,7\n"
+      "movq rdx,-3\n"
+      "imulq (rax,rdx),rdx\n"
+      "addq rax,rdx\n"
+      "ret\n");
+}
+
 ASSEMBLER_TEST_GENERATE(SignedMultiply64, assembler) {
   __ pushq(R15);  // Callee saved.
   __ movq(RAX, Immediate(2));