That is just a boolean check and a single function call. It won't make much of a difference in terms of performance. When you call flip h, if you are passing the same value then nothing happens visually (only if the value changes). So there is essentially no cost. I mean, there is still some cost, but you could easily make thousands of calls before you could measure any drop in the FPS.
Also, you could do this to collapse it to one line (this is technically faster, but you could never notice):
$PlayerSprite.set_flip_h(input_vector.x < 0)
However, using $ (or get_node) could be a performance issue with lots of calls. So you should put this at the top of the script.
onready var player_sprite = $PlayerSprite
Then do this:
player_sprite.set_flip_h(input_vector.x < 0)