Don't count nested function scopes towards MaxNestedContextChainlength

This is allocating registers in the function for all inner contexts that can be active in that function, so that nested blocks always have O(1) access to all outer contexts. However, currently it's always walking into nested functions, overallocating the number of registers, causing additional register pressure.

BUG=v8:5484

Review-Url: https://codereview.chromium.org/2408303003
Cr-Commit-Position: refs/heads/master@{#40214}

src/ast/scopes.cc

@@ -1153,6 +1153,7 @@ int Scope::ContextChainLengthUntilOutermostSloppyEval() const {
 int Scope::MaxNestedContextChainLength() {
   int max_context_chain_length = 0;
   for (Scope* scope = inner_scope_; scope != nullptr; scope = scope->sibling_) {
+    if (scope->is_function_scope()) continue;
     max_context_chain_length = std::max(scope->MaxNestedContextChainLength(),
                                         max_context_chain_length);
   }