Deoptimization is easier to diagnose when there is a text reason.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index cc635100b18dcab10c9e4e9c02aee8ab50206464..5b6bd34d1a36b2475929052b6f9b723a180b6934 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -5447,6 +5447,8 @@ void LCodeGen::DoDeoptimize(LDeoptimize* instr) {
   if (info()->IsStub() && type == Deoptimizer::EAGER) {
     type = Deoptimizer::LAZY;
   }
+
+  Comment(";;; deoptimize: %s", instr->hydrogen()->reason());
   DeoptimizeIf(no_condition, instr->environment(), type);
 }