[turbofan] Add --csa-verify flag that enables machine graph verification of code stubs.

src/flag-definitions.h

Index: src/flag-definitions.h
diff --git a/src/flag-definitions.h b/src/flag-definitions.h
index 60ff4fccc9324df889d1689e357b493fa7ec1cbc..70d81a8c7059173b5d38cd8a6fb5be7bb637a740 100644
--- a/src/flag-definitions.h
+++ b/src/flag-definitions.h
@@ -444,6 +444,7 @@ DEFINE_BOOL(turbo_asm_deoptimization, false,
 DEFINE_BOOL(turbo_verify, DEBUG_BOOL, "verify TurboFan graphs at each phase")
 DEFINE_STRING(turbo_verify_machine_graph, nullptr,
               "verify TurboFan machine graph before instruction selection")
+DEFINE_INT(csa_verify, 0, "verify first N TurboFan code stubs")
 DEFINE_BOOL(turbo_stats, false, "print TurboFan statistics")
 DEFINE_BOOL(turbo_stats_nvp, false,
             "print TurboFan statistics in machine-readable format")