Fixes needed by screen-13

[attackgoat]

I found these two issues testing attackgoat/screen-13#68, however I'm not sure they are done in the best way yet.

Commit e8c5d00

Fixes a panic at spirq/src/reflect.rs:621.

Valid SPIR-V may list function body instructions after their first call. The function type declaration of course always happens before the first call. This can be reproduced with the following shader using shader-reflect:

#version 460 core

vec4 a() {
    return vec4(0);
}

void main() {
    gl_Position = a();
}

Commit 7ee6d99

Specialization constants that are found via ReflectConfig::specialize(..) are ConstantValue::Typeless(_) and remain untyped after processing their spec-const instructions. The typeless state is helpful because Vulkan doesn't require that information from users and so it is very handy to let spirq reflect that info automatically.

[PENGUINLIONG]

Thank you! For the first problem we might better track function calls in an independent dictionary. I can work on that in another PR. Let's merge this first. :)

[PENGUINLIONG]

Turns out the function call logic is wrong. Gonna fix it right away. 🙃