Avoid excess breaks within switch (fix #5344)

[edemaine]

• Fix switch generates unneeded breaks #5344: no longer generate break when the case body exits on its own, typically by a forced return being pushed within if blocks.
• Add new alwaysJumps node method to detect when a switch case can
  safely drop its break. Similar to jumps but ANDing instead of OR.
• Fix some documentation describing jumps, which ignores throw; it turns out x = (throw 1) is valid CoffeeScript.