Some more GDScript performance optimizations

[hpvb]

Two new changes to GDScript:

• On compilers that support computed goto use it to dispatch opcodes in GDScript::call()
• Remove several indexing checks on DEBUG_RELEASE, these shouldn't appear in released games.

And an optimization for safe_refcount:

• Allow better inlining of safe_refcount implementations. Not a huge win but very little effort and the effect is measurable.

Overall improvement is about 20% for the following GDScript

func _ready():
	var time_before = OS.get_ticks_msec()

	var c
	for i in range(10000000):
		var next 
		var first = 1
		var second = 2
		for c in range(40):
			next = first + second
			first = second
			second = next
		c = next
	print(c)

	var total_time = OS.get_ticks_msec() - time_before
	print("Time taken: " + str(total_time))
	get_tree().quit()
	pass

Regression testing has been done with 2d/platformer, 3d/platformer, goltorus, and this program.

[hpvb]

@karroffel @reduz @akien-mga @bojidar-bg I think this is done now. It'd be great if one of you could have a look and merge if you see no problems.

I'm working on another PR to optimize the most common operations based on the addressing modes of the parameters. Once that's done for 3.0 I think these types of optimizations are done. I think I could eek out another 10 - 15% by making many more 1 - 2% changes but I'm not entirely sure if this is worth doing.