[turbofan] Fix printing of parameter registers in serializer trace

Bug: v8:7790
Change-Id: Ibfc83828c8677901caa4e04e2b88915ddabeed49
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1918245
Auto-Submit: Georg Neis <neis@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64984}

src/compiler/serializer-for-background-compilation.cc

@@ -889,7 +889,11 @@ std::ostream& operator<<(
       Hints const& hints = env.ephemeral_hints_[i];
       if (!hints.IsEmpty()) {
         if (i < env.parameter_count()) {
-          output_stream << "Hints for a" << i << ":\n";
+          if (i == 0) {
+            output_stream << "Hints for <this>:\n";
+          } else {
+            output_stream << "Hints for a" << i - 1 << ":\n";
+          }
         } else if (i < env.parameter_count() + env.register_count()) {
           int local_register = i - env.parameter_count();
           output_stream << "Hints for r" << local_register << ":\n";