Autocomplete no longer recognizes type under if-block checked by 'is'

[ghost]

Godot version:

af290f7

OS/device including version:

Linux

Issue description:

A type's autocompletion would appear under an if-block when that object's type has been checked; for a common example:

func _input(event):
    if event is InputEventKey:
        event.

In 3.0.6, autocomplete would give you this:

Post-typed gdscript, this no longer works; the autocompletion for event under the if-block just displays the following:

Using typed gdscript, autocomplete works fine outside of the if-block (albeit just for the base class):

func _input(event : InputEvent) -> void:
    event.
    if event is InputEventKey:

But inside the if-block, again, it displays new and some constants.

As of right now, it's necessary for the user to cast event before autocomplete will appear:

var key = event as InputEventKey
if key:

[akien-mga]

CC @vnen

[ghost]

Been noticing this as well. Wasn't sure about it though, since I had some other auto-completion issue.