Add Chapter - OpUndef

Similar to the LLVM `undef` (and later `Poison`)

What is `OpUndef`, when is it used, when can it be valid (or invalid)

```
%select = OpSelect %int %cond %undef %value
%access = OpAccessChain %ptr_uniformconstant_image %image_array %select
%load = OpLoad %image_ty %access
%fetch = OpImageFetch %v4f32 %load %coords Lod 0
```