Split GuardField into GuardFieldType and GuardFieldLength instructions.

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
diff --git a/runtime/vm/flow_graph_compiler.cc b/runtime/vm/flow_graph_compiler.cc
index 063415f769606502262b5a853cd6b2f9f6fbd65c..3a6c97e345464d2b5d14f7e5e243ebb00485a792 100644
--- a/runtime/vm/flow_graph_compiler.cc
+++ b/runtime/vm/flow_graph_compiler.cc
@@ -256,11 +256,10 @@ void FlowGraphCompiler::EmitInstructionPrologue(Instruction* instr) {
       AddCurrentDescriptor(PcDescriptors::kDeopt,
                            assert->deopt_id(),
                            assert->token_pos());
-    } else if (instr->IsGuardField() ||
-               (instr->CanBecomeDeoptimizationTarget() && !instr->IsGoto())) {
-      // GuardField and instructions that can be deoptimization targets need
-      // to record their deopt id.  GotoInstr records its own so that it can
-      // control the placement.
+    } else if (instr->CanBecomeDeoptimizationTarget() && !instr->IsGoto()) {
+      // Instructions that can be deoptimization targets need to record kDeopt
+      // PcDescriptor corresponding to their deopt id. GotoInstr records its
+      // own so that it can control the placement.
       AddCurrentDescriptor(PcDescriptors::kDeopt,
                            instr->deopt_id(),
                            Scanner::kNoSourcePos);