[turbofan] Introduces a step to verify the machine graph.

src/flag-definitions.h

Index: src/flag-definitions.h
diff --git a/src/flag-definitions.h b/src/flag-definitions.h
index f906aa9b180aa151d671deabf9a5bfb8be279e39..401e9b039aedca4d439f1e4cbdb53f7c0003fa39 100644
--- a/src/flag-definitions.h
+++ b/src/flag-definitions.h
@@ -453,6 +453,8 @@ DEFINE_BOOL(turbo_asm, true, "enable TurboFan for asm.js code")
 DEFINE_BOOL(turbo_asm_deoptimization, false,
             "enable deoptimization in TurboFan for asm.js code")
 DEFINE_BOOL(turbo_verify, DEBUG_BOOL, "verify TurboFan graphs at each phase")
+DEFINE_BOOL(turbo_verify_machine_graph, false,
+            "verify TurboFan machine graph before instruction selection")
 DEFINE_BOOL(turbo_stats, false, "print TurboFan statistics")
 DEFINE_BOOL(turbo_stats_nvp, false,
             "print TurboFan statistics in machine-readable format")