Fix -Wunreachable-code-aggressive warning in debug build

Bug: chromium:1066980
Change-Id: I4e149f5dd9ac2d3b8f51afd2c63451d509441e64
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3110652
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76413}

src/diagnostics/arm64/eh-frame-arm64.cc

@@ -54,7 +54,6 @@ const char* EhFrameDisassembler::DwarfRegisterCodeToString(int code) {
       return "sp";  // This could be zr as well
     default:
       UNIMPLEMENTED();
-      return nullptr;
   }
 }