[cleanup] Fix warnings reported by clang-tidy

Fixes several warnings reported for internal repo by:
  * using vector::empty instead of vector::size() == 0
  * removing redundant return; at the end of a function
  * making operator= return OriginalType&

Bug: v8:9183
Change-Id: I8c725bd7b0bc011557fb2bb68a561ee413ab38f5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1589978
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61149}

src/compilation-statistics.cc

@@ -84,7 +84,7 @@ static void WriteLine(std::ostream& os, bool machine_format, const char* name,
         stats.max_allocated_bytes_, stats.absolute_max_allocated_bytes_);
 
     os << buffer;
-    if (stats.function_name_.size() > 0) {
+    if (!stats.function_name_.empty()) {
       os << "   " << stats.function_name_.c_str();
     }
     os << std::endl;

src/feedback-vector.cc

@@ -568,7 +568,7 @@ bool FeedbackNexus::ConfigureMegamorphic(IcCheckType property_type) {
 Map FeedbackNexus::GetFirstMap() const {
   MapHandles maps;
   ExtractMaps(&maps);
-  if (maps.size() > 0) return *maps.at(0);
+  if (!maps.empty()) return *maps.at(0);
   return Map();
 }
 

src/objects.cc

@@ -7506,7 +7506,6 @@ void CompilationCacheTable::Remove(Object value) {
       ElementRemoved();
     }
   }
-  return;
 }
 
 template <typename Derived, typename Shape>

src/objects/string.cc

@@ -295,7 +295,6 @@ void String::StringShortPrint(StringStream* accumulator, bool show_details) {
     }
     if (show_details) accumulator->Put('>');
   }
-  return;
 }
 
 void String::PrintUC16(std::ostream& os, int start, int end) {  // NOLINT

src/utils.cc

@@ -243,7 +243,7 @@ std::vector<char> ReadCharsFromFile(const char* filename, bool* exists,
 }
 
 std::string VectorToString(const std::vector<char>& chars) {
-  if (chars.size() == 0) {
+  if (chars.empty()) {
     return std::string();
   }
   return std::string(chars.begin(), chars.end());

src/utils.h

@@ -574,9 +574,9 @@ class SetOncePointer {
     pointer_ = value;
   }
 
-  T* operator=(T* value) {
+  SetOncePointer& operator=(T* value) {
     set(value);
-    return value;
+    return *this;
   }
 
   bool operator==(std::nullptr_t) const { return pointer_ == nullptr; }