Adds load/store from signed unscaled 9-bit offset w/ no writeback.

runtime/vm/simulator_arm64.cc

Index: runtime/vm/simulator_arm64.cc
===================================================================
--- runtime/vm/simulator_arm64.cc	(revision 35221)
+++ runtime/vm/simulator_arm64.cc	(working copy)
@@ -1321,6 +1321,12 @@
     const uint32_t imm12 = static_cast<uint32_t>(instr->Imm12Field());
     const uint32_t offset = imm12 << size;
     address = rn_val + offset;
+  }  else if (instr->Bits(10, 2) == 0) {
+    // addr = rn + signed 9-bit immediate offset.
+    wb = false;
+    const int64_t offset = static_cast<int64_t>(instr->SImm9Field());
+    address = rn_val + offset;
+    wb_address = rn_val;
   } else if (instr->Bit(10) == 1) {
     // addr = rn + signed 9-bit immediate offset.
     wb = true;