Improve performance for scope_for_create

```ruby
class Post < ActiveRecord::Base
end

posts = Post.where(id: 1, title: "foo")

Benchmark.ips do |x|
  x.report("scope_for_create") { posts.scope_for_create }
end
```

Before:

```
Warming up --------------------------------------
    scope_for_create    30.125k i/100ms
Calculating -------------------------------------
    scope_for_create    334.033k (± 4.4%) i/s -      1.687M in   5.060493s
```

After:

```
Warming up --------------------------------------
    scope_for_create    35.088k i/100ms
Calculating -------------------------------------
    scope_for_create    388.603k (±11.8%) i/s -      1.930M in   5.080998s
```

activerecord/lib/active_record/relation.rb

@@ -686,7 +686,9 @@ module ActiveRecord
     end
 
     def scope_for_create
-      where_values_hash.merge!(create_with_value.stringify_keys)
+      hash = where_values_hash
+      create_with_value.each { |k, v| hash[k.to_s] = v } unless create_with_value.empty?
+      hash
     end
 
     # Returns true if relation needs eager loading.

activerecord/lib/active_record/relation/where_clause.rb

@@ -47,18 +47,12 @@ module ActiveRecord
       end
 
       def to_h(table_name = nil)
-        equalities = equalities(predicates)
-        if table_name
-          equalities = equalities.select do |node|
-            node.left.relation.name == table_name
-          end
-        end
-
-        equalities.map { |node|
+        equalities(predicates).each_with_object({}) do |node, hash|
+          next if table_name&.!= node.left.relation.name
           name = node.left.name.to_s
           value = extract_node_value(node.right)
-          [name, value]
-        }.to_h
+          hash[name] = value
+        end
       end
 
       def ast