Add check for uninitialized variables being used

We hit in issue in [GPU-AV](https://github.com/KhronosGroup/Vulkan-ValidationLayers/pull/9471#discussion_r2004733025) where we basically had something like https://godbolt.org/z/McxnoT4no

and found that trying to index into an array with an uninitialized `Private OpVariable` causes a driver to crash, but when doing a `if (cache == 0)` or `printf("%u", cache)` on it, it would then be zero

Part of this is a problem that glslang doesn't allow going

```
%uint_0 = OpConstant %uint 0
%_ptr_Private_uint = OpTypePointer Private %uint
%cache = OpVariable %_ptr_Private_uint Private %uint_0
```

but this is something that could easily trip up a real developer. Would be nice to add a [proper VUID](https://gitlab.khronos.org/vulkan/vulkan/-/issues/4222) and check for this in GPU-AV in the future