Reland "[parser] Simplify ParseFunctionBody"

This is a reland of 2963f1b2

Original change's description:
> [parser] Simplify ParseFunctionBody
> 
> - Merge is_simple branches at the top
> - Remove block around inner_body parsing. Always merge fully at the end.
> - Remove conditional inner block adding to outer body. Simply add it to the
>   inner body making merge push it to the parent.
> 
> Change-Id: I1f062918a7abac354b949136463517bd0440984f
> Reviewed-on: https://chromium-review.googlesource.com/c/1386111
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#58390}

Change-Id: I145f0cb1eda1dca4dd047b55e54b2b1bb704ecf8
Reviewed-on: https://chromium-review.googlesource.com/c/1397662Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58567}

src/parsing/parser-base.h

@@ -3817,9 +3817,13 @@ void ParserBase<Impl>::ParseFunctionBody(
     const FormalParametersT& parameters, FunctionKind kind,
     FunctionLiteral::FunctionType function_type, FunctionBodyType body_type) {
   if (IsResumableFunction(kind)) impl()->PrepareGeneratorVariables();
+
+  DeclarationScope* function_scope = parameters.scope;
+  DeclarationScope* inner_scope = function_scope;
+
   // Building the parameter initialization block declares the parameters.
   // TODO(verwaest): Rely on ArrowHeadParsingScope instead.
-  if (!parameters.is_simple) {
+  if (V8_UNLIKELY(!parameters.is_simple)) {
     if (has_error()) return;
     BlockT init_block = impl()->BuildParameterInitializationBlock(parameters);
     if (IsAsyncFunction(kind) && !IsAsyncGeneratorFunction(kind)) {
@@ -3827,20 +3831,13 @@ void ParserBase<Impl>::ParseFunctionBody(
     }
     body->Add(init_block);
     if (has_error()) return;
-  }
-
-  DeclarationScope* function_scope = scope()->AsDeclarationScope();
-  bool allow_duplicate_parameters = false;
-  BlockT inner_block = impl()->NullBlock();
-  {
-    StatementListT inner_body(pointer_buffer());
-    DeclarationScope* inner_scope = function_scope;
 
-    if (!parameters.is_simple) {
     inner_scope = NewVarblockScope();
     inner_scope->set_start_position(scanner()->location().beg_pos);
   }
 
+  StatementListT inner_body(pointer_buffer());
+
   {
     BlockState block_state(&scope_, inner_scope);
 
@@ -3884,7 +3881,9 @@ void ParserBase<Impl>::ParseFunctionBody(
 
   scope()->set_end_position(end_position());
 
-    if (parameters.is_simple) {
+  bool allow_duplicate_parameters = false;
+
+  if (V8_LIKELY(parameters.is_simple)) {
     DCHECK_EQ(inner_scope, function_scope);
     if (is_sloppy(function_scope->language_mode())) {
       impl()->InsertSloppyBlockFunctionVarBindings(function_scope);
@@ -3904,8 +3903,9 @@ void ParserBase<Impl>::ParseFunctionBody(
 
     inner_scope->set_end_position(end_position());
     if (inner_scope->FinalizeBlockScope() != nullptr) {
-        inner_block = factory()->NewBlock(true, inner_body);
+      BlockT inner_block = factory()->NewBlock(true, inner_body);
       inner_body.Rewind();
+      inner_body.Add(inner_block);
       inner_block->set_scope(inner_scope);
       const AstRawString* conflict = inner_scope->FindVariableDeclaredIn(
           function_scope, VariableMode::kLastLexicalVariableMode);
@@ -3916,10 +3916,6 @@ void ParserBase<Impl>::ParseFunctionBody(
       impl()->InsertShadowingVarBindingInitializers(inner_block);
     }
   }
-    inner_body.MergeInto(body);
-  }
-
-  if (!impl()->IsNull(inner_block)) body->Add(inner_block);
 
   ValidateFormalParameters(language_mode(), parameters,
                            allow_duplicate_parameters);
@@ -3932,6 +3928,8 @@ void ParserBase<Impl>::ParseFunctionBody(
   }
 
   impl()->DeclareFunctionNameVar(function_name, function_type, function_scope);
+
+  inner_body.MergeInto(body);
 }
 
 template <typename Impl>