Arithmetic-assignment operators incorrectly set the type of the variable

[aaronfranke]

Godot version: master after the GDScript "2.0" rewrite

OS/device including version: Ubuntu 20.04

Issue description:

	var test := Vector3()
	test *= 3 # Parse Error: Cannot assign a value of type "int" to a target of type "Vector3".

The above works in Godot 3.x, but is broken in the current master branch. However, the below works in the master branch:

	var test := Vector3()
	test = test * 3

Steps to reproduce: Copy the above code, and use the current master branch.

Minimal reproduction project: I'm going to not bother with one, this is a very simple bug.