Fix some instances of -Wunreachable-code-aggressive.

Bug: chromium:1066980
Change-Id: I92570706da4e6f8380f100bec12c8414f6ff2edc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3086221
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76248}

src/diagnostics/eh-frame.cc

@@ -27,14 +27,12 @@ void EhFrameWriter::WriteInitialStateInCie() { UNIMPLEMENTED(); }
 
 int EhFrameWriter::RegisterToDwarfCode(Register) {
   UNIMPLEMENTED();
-  return -1;
 }
 
 #ifdef ENABLE_DISASSEMBLER
 
 const char* EhFrameDisassembler::DwarfRegisterCodeToString(int) {
   UNIMPLEMENTED();
-  return nullptr;
 }
 
 #endif

src/wasm/wasm-js.cc

@@ -1578,7 +1578,6 @@ void EncodeExceptionValues(v8::Isolate* isolate,
       case i::wasm::kBottom:
       case i::wasm::kS128:
         UNREACHABLE();
-        break;
     }
   }
 }