post queries: fix handling of '~' operator.

Fix queries like `(~a ~b) (~c ~d)` being handled like `~a ~b ~c ~d`.
Caused by trimming AND nodes from the tree before rewriting the '~'
operator, which caused `~a` terms to be incorrectly lifted out of
subexpressions.

app/logical/post_query.rb

@@ -13,7 +13,7 @@ class PostQuery
 
   # Return a new PostQuery with aliases replaced.
   def self.normalize(...)
-    PostQuery.new(...).replace_aliases.trim
+    PostQuery.new(...).replace_aliases.rewrite_opts.trim
   end
 
   def initialize(search_or_ast, current_user: User.anonymous, tag_limit: nil, safe_mode: false, hide_deleted_posts: false)
@@ -129,6 +129,11 @@ class PostQuery
     build(ast.trim)
   end
 
+  # Return a new PostQuery with the '~' operator replaced with OR clauses.
+  def rewrite_opts
+    build(ast.rewrite_opts)
+  end
+
   # Return a new PostQuery with aliases replaced.
   def replace_aliases
     return self if aliases.empty?