split-if-assignments should handle `else if` assignments

[GoodForOneFare]

Previously, else if (foo = bar) would throw an exception because the transform tried to move the assignment to before the else. That would've created an invalid AST.

This updates the logic to rewrite else if (foo = bar) {} to

else { 
  foo = bar; 
  if (foo) {

  }
}

Please review @lemonmade, @TylerHorth

[GoodForOneFare]

@lemonmade please take another look. I've cleaned up indentation, and used path.replace instead of direct node assignment.