[turbofan] Implement jump threading after register allocation.

src/flag-definitions.h

Index: src/flag-definitions.h
diff --git a/src/flag-definitions.h b/src/flag-definitions.h
index 4d22d838bb62f2cc552c182cfaca064e57d1488a..5d01ebbdd217b52de321968dcfb6970df3b0945d 100644
--- a/src/flag-definitions.h
+++ b/src/flag-definitions.h
@@ -373,6 +373,7 @@ DEFINE_STRING(trace_turbo_cfg_file, NULL,
 DEFINE_BOOL(trace_turbo_types, true, "trace TurboFan's types")
 DEFINE_BOOL(trace_turbo_scheduler, false, "trace TurboFan's scheduler")
 DEFINE_BOOL(trace_turbo_reduction, false, "trace TurboFan's various reducers")
+DEFINE_BOOL(trace_turbo_jt, false, "trace TurboFan's jump threading")
 DEFINE_BOOL(turbo_asm, false, "enable TurboFan for asm.js code")
 DEFINE_BOOL(turbo_verify, false, "verify TurboFan graphs at each phase")
 DEFINE_BOOL(turbo_stats, false, "print TurboFan statistics")
@@ -395,6 +396,7 @@ DEFINE_BOOL(turbo_reuse_spill_slots, false, "reuse spill slots in TurboFan")
 // TODO(dcarney): this is just for experimentation, remove when default.
 DEFINE_BOOL(turbo_delay_ssa_decon, false,
             "delay ssa deconstruction in TurboFan register allocator")
+DEFINE_BOOL(turbo_jt, true, "enable jump threading")
 
 DEFINE_INT(typed_array_max_size_in_heap, 64,
            "threshold for in-heap typed array")