Use a uniform way to emit code for all instructions.

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
diff --git a/runtime/vm/flow_graph_compiler_x64.cc b/runtime/vm/flow_graph_compiler_x64.cc
index dd764eac913b1f0d3de647199ad2b6d46d852895..935aa9ae005ddfb983c3156a7d80a8bda351ff63 100644
--- a/runtime/vm/flow_graph_compiler_x64.cc
+++ b/runtime/vm/flow_graph_compiler_x64.cc
@@ -724,12 +724,7 @@ void FlowGraphCompiler::EmitInstructionPrologue(Instruction* instr) {
       AddCurrentDescriptor(PcDescriptors::kDeopt,
                            assert->deopt_id(),
                            assert->token_pos());
-    } else if (instr->IsGuardField()) {
-      GuardFieldInstr* guard = instr->AsGuardField();
-      AddCurrentDescriptor(PcDescriptors::kDeopt,
-                           guard->deopt_id(),
-                           Scanner::kDummyTokenIndex);
-    } else if (instr->CanBeDeoptimizationTarget()) {
+    } else if (instr->CanBeDeoptimizationTarget() || instr->IsGuardField()) {
       AddCurrentDescriptor(PcDescriptors::kDeopt,
                            instr->deopt_id(),
                            Scanner::kDummyTokenIndex);