Optimize AOT's switchable calls for the monomorphic case.

runtime/vm/instructions_arm64_test.cc

Index: runtime/vm/instructions_arm64_test.cc
diff --git a/runtime/vm/instructions_arm64_test.cc b/runtime/vm/instructions_arm64_test.cc
index 970d271b4cdfc3cd47e70d25546851536bb5c45a..7ad4fb9b6ea97ca3be7250b328e0c7ddaa8b444c 100644
--- a/runtime/vm/instructions_arm64_test.cc
+++ b/runtime/vm/instructions_arm64_test.cc
@@ -27,7 +27,8 @@ ASSEMBLER_TEST_RUN(Call, test) {
   // The return address, which must be the address of an instruction contained
   // in the code, points to the Ret instruction above, i.e. one instruction
   // before the end of the code buffer.
-  CallPattern call(test->entry() + test->code().Size() - Instr::kInstrSize,
+  uword end = test->payload_start() + test->code().Size();
+  CallPattern call(end - Instr::kInstrSize,
                    test->code());
   EXPECT_EQ(StubCode::InvokeDartCode_entry()->code(),
             call.TargetCode());